Field of the Invention

The present invention relates to lubricating compositions, particularly to lubricating compositions for use in engines operated under sustained high load conditions, such as marine diesel engines and power applications . Background of the Inyention

Lubricating oils for use in internal combustion engines are subject to high levels of stress. It is essential that the lubricating oil provides good lubrication properties under a varie-y of conditions, and amongst other properties, should provide good wear, corrosion protection, help to keep the engine clean, be thermally and oxidatively stable and carry heat away from the engine.

Lubricating oils used in marine diesel engines are subject to particularly high levels of stress due to the fact that marine engines are usually run continuously at near full load conditions for long periods of time, often in remote locations. In addition, the lubricating oils are expected to have long lifetimes since there is often little or no opportunity for changing the lubricating oils in a marine engine.

It will be appreciated in the art that the term "marine" does not restrict such engines to those used in water-borne vessels. That is to say, in addition said term also includes engines used for power generation applications. These highly rated, fuel efficient, slow- and medium-speed marine and stationary diesel engines operate at high pressures, high temperatures and long- strokes.

The ageing performance and oxidation stability is one of the most important properties of trunk piston engine oil, since the exposure time of t.he oil in an engine coul d be several thousands of hours and modern engines can subject an oil to high levels of thermal stress .

It has now surprisingly been found that by incorporating into the lubricating composition a Fischer- Tropsch derived base oil having a kinematic viscosity at 100 'C in the range of from 15 to 30 W/s, together with one or more additives, a significant improvement in the oxidative stability of the lubricating compositions is observed.

Summary of the Invention

According to the present invention there is provided a lubricating composition comprising: (i) one or more additives; (ii) at least 5 % by weight of a Fischer-Tropsch derived heavy base oil having a kinematic viscosity at 100 °C in the range of from 15 mrnVs to 30 mm ² /s; wherein the lubricating composition has a TBN of at least 10 mg KOH/g and a kinematic viscosity at 100 "C of at least 9 icmVs.

Λs well as having excellent lubricaLing properties, the lubricating composition provides improved oxidative stability.

Hence according to another aspect of the present invention there is provided the use in a lubricating composition of at least 5 % by weight of a Fischer- Tropsch derived heavy base oil having a kinematic viscosity at 100 "C in the range of from 15 to 30 mmVs for increasing the oxidation stability of the lubricating composition.

According to yet another aspect of the present invention there is provided the use in a lubricating composition of at least 5 % by weight of composition of a Fischei—Tropsch derived heavy base oil having a kinematic viscosity at 100 °C in the range of from 15 to 30 mmVs for a 4-stroke marine engine. Detailed Description of the Invention The term "Fischer-Tropsch derived" as used herein means that a material is, or derives from, a synthesis product of a Fischer-ϊropsch condensation process. A Fischer-Tropsch derived product may also be referred to as a "GTL (Gas-to-Liquid) " product. A first essential component oi the lubricating compositions herej n is a Fischer-Tropsch derived paraffinic base oil having a kinematic viscosity at 100 'C in the range of from 15 to 30 mπι ² /s (herinafter referred to as a Fischer-Tropsch derived heavy base oil) . The Fischer-Tropsch derived heavy case oil is present at a level of at least 5%, preferably at least 10%, more preferably at least 15%, by weight of the lubricating composition. Higher levels of the Fischer- ϊropsch derived heavy base oil may also be used in the lubricating compositions of the invention, typically, for example, at least 20%, more preferably at least 30%, even more preferably at least 35%, by weight of composition. The Fischer-Tropsch derived heavy base oil is preferably present at a level of at most 90%, more preferably at a level of at most 70% and even more preferably at a level of at most 50%.

The Fischer-Tropsch derived paraffinic base oil preferably has a kinematic viscosity at 100° C in the range of from 15 Lo 30 mmVs, preferably in the range of from 18 zo 27 mmVs. In one embodiment of the present invention, the Fischer-Tropsch derived heavy base oil has a kinematic viscosity at 100° C in the range of from 18 to 2? mm ² /s, as exemplified by "GTL 19" (having a kinematic viscosity at 10CTC of 19 cSt. In another embodiment of the present invention, the Fischer-Tropsch derived heavy base oil has a kinematic viscosity at 100 "C in the range of from 24 to 27 mm ² /s, as exemplified by "GTL 26" (having a kinematic viscosity at 100°C of 26 cSt) .

In the context of the present invention, a Fischer- Tropsch derived paraffinic heavy base oil is suitably a base oil which has been derived, whether directly or indirectly following one or more downstream processing steps, from a Fischer-Tropsch "bottoms" (i.e. high boiling) product. A Fischer-Tropsch bottoms product is a hydrocarbon product recovered from the bottom of a fractionation column, usjally a vacuum column, following fractionation of a Fischer-Tropsch derived feed stream. More preferably the paraffinic base oil is prepared by hydroisomerisalion of a paraffin wax, as prepared in a Fischer-Tropsch synlhesis step, and dewaxing a residual fraction as separated from the effluent of said hydroisomerisation process. Examples of such processes suitable for preparing the paraffinic base oils are described in WO-A-2004/007647, US-A-US2004/0065588, WO-A- 2004/033595 and WO-A-02070627, which publications are hereby incorporated by reference.

The relatively heavy feed to the hydroisomerisating step has suitably a weight ratio of compounds having at least 60 or more carbon atoms and compounds having at least 30 carbon atoms of at least 0.2, preferably at least 0.4 and more preferably at least 0.55. Furthermore the feed has at least 30 wt%, preferably at leasi: 50 wt% and more preferably at least 55 wt% of compounds having at least 30 carbon atoms. Such a feed preferably comprises a Fischer-Tropsch product, which in turn comprises a C20+ fraction having an ASF-alpha value

[Anderson-Schu] z-Flory chain growth factor) of at least 0.925, preferably at least 0.935, more preferably at least 0.945, even more preferably at least 0.955. The initial boiling point of the feed is preferably below 200 ⁰ C. Preferably any compounds having 4 or less carbon atoms and any compounds having a boiling point in that range are not present in said feed. The feed may also comprise process recycles and/or off-spec base oil fractions as obtained after dewaxing. A suitable Fischer-Tropsch synthesis process, which may yield a relatively heavy Fischer-Tropsch product, is for example described in VIO-A-9934917.

The process will generally comprise a Fischer- Tropsch synthesis to obtain a Fischer-Tropsch wax, a hydroisomerisation step and a pour point reducing step of a residual fraction, comprising

(a) hydrocracking/hydroisomerising a Fischer-Tropsch wax,

(b) separating from the product of step (a) a distillation residue and

(c) dewaxing the distillation residue to obtain the paraffinic base oil; and op-ionally

(d) a re-distillation of the paraffinic base oil to remove light ends such to obtain a residual paraffinic base oil having the desired viscosity.

The hydroconversion/hydroisomerisation reaction of step (a) is preferably performed in the presence of hydrogen and a catalyst, which catalyst can be chosen from those known to one ski.1 led in the art as being suitable for this reaction of which some will be described in more detail below. The catalyst may in principle be any catalyst known in the art to be suitable for isomerising paraffinic molecules. In general, suitable hydroconversion/hydroisomerisation catalysts are those comprising a hydrogenation component supported on a refractory oxide carrier, such as amorphous silica- alumina (ASA) , alumina, fluorided alumina, molecular sieves (zeolites) or mixtures of two or more of these. One type of preferred catalysts to be applied in the hydroconversion/hydroisomerisation step in accordance with the present invention are hydroconversion/ hydro.i somerisatioπ catalysts comprising platinum and/or palladium as the hydrogenation component. A very much preferred hydroconversion/hydroisomerisation catalyst comprises platinum and palladium supported on an amorphous silica-alumina {ASA) carrier. The platinum and/or palladium is suitably present in an amount of from 0.1 to 5.0% by weight, more suitably from 0.2 to 2.0% by weight, calculated as element and based on total weight of carrier. If both present, the weight ratio of platinum to palladium may vary within wide limits, but suitably is in the range of from 0.05 to 10, more suitably 0.1 to 5. Examples of suitable noble metal on ASA catalysts are, for instance, disclosed in WO-A-9410264 and EP-A-0582347. Other suitable noble metal-based catalysts, such as platinum on a fluorided alumina carrier, are disclosed in e.g. US-A-5059299 and WO-A-9220759. A second type of suitable hydrocor.version/ hydroisomerisation catalysts are those comprising at least one Group VIB metal, preferably tungsten and/or molybdenum, and at least one non-noble Group vTIT metal, preferably nickel and/or cobalt, as the hydrogenation component. Both metals may be present as oxides, sulphides or a combination thereof. The Group VTB metal is suitably present in an amount of from 1 to 35% by weight, more suitably from 5 to 30% by weight, calculated as element, and based on total weight of the carrier. The non-noble Group VIII metal is suitably present in an amount of from 1 to 25 wt%, preferably 2 to 15 wt%, calculated as element and based on total weight of carrier, h hydroconversion catalyst of this type, which has been found particularly suitable, is a catalyst comprising nickel and rungsten supported on fluor ^J ded alumina .

The above non-noble metal-based catalysts are preferably used in their sulphided form. In order to maintain the sulphided form of the catalyst during use some sulphur needs to be present in the feed. Prelerably ar least 10 mg/kg and more preferably between 50 and 150 mg/kg of sulphur is present in the feed. A preferred catalyst, which can be used in a non- sulphideα form, comprises a non-noble Group VIII metal, e.g., iron, nickel, in conjunction with a Group IB metal, e.g., copper, supported on an acidic support. Copper is preferably present to suppress hydrogeiiolysis of paraffins to methane. The catalyst has a pore volume preferably in the range of 0.35 to 1.10 ml/g as determined by water absorption, a surface area of preferably between 200-500 m ² /g as determined by BET nitrogen adsorption, and a bulk density of between 0.4-1.0 g/ml. The catalyst support is preferably made of an amorphous silica-alumina wherein the alumina may be present within wide range of between 5 and 96 wt%, preferably between 20 and 85 wt%. The silica content as Siθ ₂ is preferably between 15 and 80 wt% . Also, the support may contain small amounts, e.g., 20-30 wt%, of a binder, e.g., alumina, silica, Group TVA metal oxides, and various types of clays, magnesia, etc., preferably alumina or silica.

The preparation of amorphous silica-alumina microspheres has been described in Ryland, Lloyd B., Tamele, M. VI. _f and Wilson, J.N., Cracking Catalysts, Catalysis: volume VII, Ed. Paul H. Ξmmett, Rexnhold Publishing Corporation, New York, 1960, pp. 5-9.

The catalyst is prepared by co-impregnating zhe metals from solutions onto the support, drying at 100-150 ⁰ C, and calcining in air at 200-550 ⁰ C. The Group VlII metal is present in amounts of about 15 wt% or less, preferably 1-1? wt%, while the Group IB metal is usually present in lesser amounts, e.g., 1:2 to about 1:20 weight ratio respecting the Group VIII metal.

A typical catalyst is shown below:

Ki, wt% 2.5-3.5 Cu, wt% 0.25-0.35

Al2θ3-Siθ2 wt% 65-75

AI2O3 (binder) wt% 25-30

Surface Area 290-325 m ² /g

Pore Volume (Hg) 0.35-0.45 ml/g Bulk Density 0.58-0.68 g/ml

Another class of suitable hydroconversion/ hydroisomerisation catalysts are tnose based on zeolitic materials, suitably comprising at least one Group VIII metal component, preferably Pt and/or Pd, as the hydrogenation component. Suitable zeolitic and other aluminosil icate materials, then, include Zeolite beta. Zeolite Y, OJtra Stable Y, ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48, MCM-68, 7.SM-35, SSZ-32, ferrierite, mordenite and silicα-aluminophosphates, such as SAPO-Il and SAPO-31. Examples of suitable hydroi somerisation/ hydroisomerisation catalysts are, for instance, described in WO-A-9201657. Combinations of these catalysts are also possible. Very suitable hydroconversion/ hydroisomerisation processes are those involving a first step wherein a zeolite beta based catalyst is used and a second step wherein a ZSM-5, ZSM-12, ZSK-22, ZSM-23, ZSM-43, KCM-68, ZSM-35, SSZ-32, ferrierile, mordenite based catalyst is used. Of the latter group ZSM-23, ZSM-22 and ZSM-48 are preferred. Examples of such processes are described in US-A-2004/0065581 and US-A-2004/0065588. In the process of US-A-2004/0065588 steps (a) and (c) as meant in the context of the present description are performed using the same ZSM-48 based catalyst.

Combinations wherein the Fischer-Tropsch product is first subjected to a first hydroisomerisation step using the amorphous catalyst comprising a silica-alumina carrier as described above followed by a second hydroisomerisation step using the catalyst comprising the molecular sieve has also been identified as a preferred process to prepare the base oi 1 to be used in the present invention. More preferred the first and second hydroisomerisation steps are performed in series flow.

In step (a) the feed is contacted with hydrogen in the presence of the catalyst at elevated temperature and pressure. The temperatures typically will be in the range of from 175 to 380 ⁰ C, preferably higher than 250 ⁰ C and more preferably from 300 to 370 ⁰ C. The pressure will typically be in the range of from 10 to 250 bar and preferably between 20 and 80 bar. Hydrogen may be supplied at a gas hourly space velocity of from 100 to 10000 Nl/l/hr, preferably from 500 to 5000 Nl/l/hr. The hydrocarbon feed may be provided at a weight hourly space velocity of from 0.1 to 5 kg/l/hr, preferably higher than 0.5 kg/l/hr and more preferably lower than 2 kg/l/hr. The ratio of hydrogen to hydrocarbon feed may range from 100 to 5000 Nl/kg and is preferably from 250 to 2500 Nl/kg.

The conversion in step {a) as defined as the weight percentage of the feed boiling above 37O ⁰ C which reacts per pass to a fraction boiling below 37C ⁰ C, is at least 20 wt%, preferably at least 25 wt%, but preferably not more than 80 wt%, more preferably not more than 65 wt%. The feed as used above in the definition is the total hydrocarbon feed fed to step (a) , thus also any optional recycle of a high boiling fraction which may be obtained in step (b) .

In step {b) a residue is isolated from the product of step (a) . With a residue is here meant that the most highest boiling compounds as present in the effluent of step (a) are part of the residue. Distillation may be performed at atmospheric pressure as illustrated in WO-A-02/070627 or lower as illustrated in WO-A-2004/007647.

Step {c) may be performed by means of solvent or catalytic dewaxing. Solvent dewaxing is advantageous because a haze free paraffinic oil may then be obtained as for example described in WO-A-0246333. A haze free base oil is defined as a composition having a cloud point of below 15°C. A hazy paraffinic base oil has a cloud point of 15°C and above. Catalytic dewaxing may yield a hazy paraffinic base oil as is illustrated in WO-A- 2004/033595 and 2004/0065588. Catalytic dewaxing is however preferred over solvent dewaxing due to its simpler oooration. Processes have therefore been developed to remove the haze from a hazy paraffinic base oil as obtained by catalytic dewaxing. Examples of said processes are US-A-6051129, US-A-2003/0075477 and US-A- 6468417. Applicants now found that when a hazy paraffinic base oil as prepared by catalytic dewaxing is used to prepare the blended oil a clear and bright product is obtained. Thus a very interesting use is found for such a hazy paraffinic base αiϋ as obtained from a Fischer- Tropsch wax. Dewaxing is preferably performed by catalytic dewaxing. Catalytic dewaxing is we] 1 known to the skilled reader and is suitaoiy performed in the presence of hydrogen and a suitable heterogeneous catalysts comprising a molecular sieve and optionally in combination with a metal having a hydrogenation function, such as the Group VITT metals. Molecular sieves, and more suitably intermediate pore size zeolites, have shown a good catalytic ability to reduce the pour point of a base oil precursor fraction under catalytic dewaxing conditions. Preferably the intermediate pore size zeolites have a pore diameter of between 0.35 and 0.8 ran. Suitable intermediate pore size zeolites are mordenite, ZSM-5, ZSM-12, ZSM-22, ZSM-23, SSZ--32, ZSM-35 and ZSM-48. Another preferred group of molecular sieves are the silica-aluminaphosphate (SAPO) materials of which SAPO-Il is most preferred as for example described in US-A-4859311. ZSM-5 may optionally be used in its HZSM-5 form in the absence of any Group VIII metal. The other molecular sieves are preferably used in combination with an added Group VIII metal. Suitable Group VIII metals are nickel, cobalt, platinum and palladium. Examples of possible combinations are Ni/ZSM-5, Pt/ZSM-23, Pd/ZSM-23, Pt/ZSM-48 and Pt/SAPO-Il. Further details and examples of suitable molecular sieves and dewaxing conditions are for example described in WO-A-9718278, US-A-5053373, US-A-5252527, US-A-4574043, US-A-5157191, WO-A-0029511, EP-A-832171. Catalytic dewaxing conditions are known in the art and typically involve operating temperatures in the range of from 200 to 500 ⁰ C, suitably from 250 to 400 ⁰ C, hydrogen pressures in the range o± from 10 to 200 bar, preferably from 40 to 70 bar, weight hourly space velocities (WHSV) in the range of from 0.1 to 10 kg of oil per litre of catalyst per hour (kg/l/hr), suitably from 0.2 to 5 kg/l/hr, more suitably from 0.5 to 3 kg/] /hr and hydrogen to oil ratios in the range of from 100 to 2,000 litres of hydrogen per litre of oil. From the effluent of step (c) the desired paraffinic base oil having the required viscosity may be directly obtained, if required any lower boiling compounds may be removed in a step (d) by distillation such to meet said viscosity requirements as specified above. A preferred process for preparing a Fischer-Tropsch derived paraffinic heavy base oil for use in the present invention is described in US-A-7354508 , the disclosure Oi which is incorporated herein by reference.

Another preferred component for use herein is & de- asphalted cylinder oil (DACO) . The de-asphalted cylinder oil may be prepared by de-asphalting a mineral-derived vacuum residue to obtain a de-asphalted oil, solvent- extracting the de-asphalted oil and obtaining the de- asphalted cylinder oil (DACO) extract. The de-asphalted cylinder oil (DACO) extract may be subjected to a solvent de-waxing step prior to being used in the present invention. Preferably, the do-asphalted cylinder oil extract is used as obtained in the solvent extraction process step without subjecting said de-asphalted cylinder oil to a de-waxing step. Further details regarding the process of preparing a de-asphalted cylinder oil can be found in EP-A-1752514, the disclosure of which is incorporated herein by reference, including the references referred to in EP-A-1752514.

When present in the lubricating compositions of the invention, the de-asphalted cylinder oil is preferably present at a level of from 0.1% to 20% by weight, more preferably from 1% to 10% by weight, and especially from 3% to 8% by weight.

The kinematic viscosity at 100° C of the de-asphalted cylinder oil is preferably at least 40 mrtι ² /s, more preferably at least 48 mπi ² /s. The pour point of the de- asphalted cylinder oil is preferably below 50°C, more preferably below 27 "C and πosL preferably below 21 "C.

The lubricating compositions of the present invention may also comprise additional base oil components in addition to the Fischer-Tropsch derived heavy base oil. Any conventional base oil suitable for use in a lubricating composition can be used herein.

Suitable base oils for use in the lubricating oil composition of the present invention include Group I, Group II or Group III base oils, polyalphaolefins, E'ischer-Tropsch derived base oils (other than those already mentioned hereinabove) and mixtures thereof.

By "Group I" base oil, "Group II" base oil and "Group III" base oil in the present invention are meant lubricating oil base oils according to the definitions of American Petroleum Institute (API) categories I, II and III. Such API categories are defined in API Publication 1509, 15th Edition, Appendix K, April 2002. Synthetic oils include hydrocarbon oils such as olefin oligomers (PAOs) , dibasic acid esters, polyol esters, and dewaxed waxy raffinate. Synthetic hydrocarbon base oils sold by the Shell Group under the designation "XHVI" (trade mark) may be convenient] y used.

In or.e embodiment of the invention, the base oil is that of mineral origin sold by the Royal Dutch/Shell Group of Companies under the designations "KVI" or "MVIN". Preferred examples of such base oils are HVI160 {commercially available under the tradename Catenex S 542) and HVI650 (a brightstock base oil, commercially available under the tradename Catenex S579) .

In a preferred embodiment of the invention, in addition to the Fischer-Tropsch derived heavy base oil, the lubricating composition comprises a Fischer-Tropsch derived base oil having a kinematic viscosity at 100 ¹ C in the range of from 6 to 10 πnn ⁷ /s, preferably in the range of from 7 to 9 ramVs, and especially about 8 mm ² /s (hereinafter referred to as a Fischer-Txopsch derived light base oil) .

The Fischer-Tropsch derived light base oil is preferably present at a level of at least 25%, more preferably at least 30%, even more preferably at least 35%, by weight of the lubricating composition. Suitable Fischer-Tropsch derived light base oils that may be conveniently used as the base oil in the lubricating composition of the present invention are those as for example disclosed in EP 0 776 959, E? 0 668 342, WO 97/21788, WO 00/15736, WO 00/14188, WO 00/14187, WO 00/14183, WO 00/14179, WO CO/08115, WO 99/41332, EP 1 029 029, WO 01/18156 and WO 01/57166.

A particularly preferred Fischer-Tropsch derived light base oil is "GTL 8". The lubricating compositions of the present invention are formulated to have a kinematic viscosity at 100 °C of at least 9 mmVs, preferably at least 10 mm ² /s, even more preferably at least 12,5 mm ² /s. The lubricating compositions of the present invention are preferably formulated to have a kinematic viscosity at 100 ⁰ C of at most 20 mm ² /s, preferably at most 18 rrtm ² /s, more preferably at most 16.3 mm ² /s.

The lubricating compositions herein are particularly suitable for use as lubricants for marine diesel engines.

Diesel engines may generally be classified as slow-speed, medium-speed or high-speed engines, with the slow-speed variety being used for the largest, deep draft vessels and in industrial applications. Slow-speed diesel engines are typically direct coupled, direct reversing, two-stroke cycle engines operating in the range of abouL 57 to 250 rpir, and usually run on residual fuels. These engines are of crosshead construction with a diaphragm and stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil.

Medium-speed engines typically operate in the range of 250 to about 1100 rpκι and may operate on the four- stroke or two-stroke cycle. These engines are trunk piston design, and many also operate on residual fuel containing in excess of 1.5 wt . % of sulphur. They may also operate on distillate fuel containing little or no residua. On deep-sea vessels these engines may bo used for propulsion, ancillary applications or both.

Slow speed and medium speed marine diesel engines are also extensively used in power plant operations. The present invention is also applicable to such applications.

Each type of diesel engine employs lubricating oils to lubricate piston rings, cylinder liners, bearings for crank shafts and connecting rods, valve train mechanisms including cams and valve lifters, among other moving members. The lubricant prevents component wear, removes heat, neutralizes and disperses combustion products, prevents rust and corrosion, and prevents sludge formation or deposits.

In low-speed marine crosshead diesel engines, the cylinders and crankcase are lubricated separately, with cylinder lubrication being provided on a once-through basis by means of injection devices that apply cylinder oil to lubricators positioned around the cylinder liner. This is known as an "all-loss" lubrication system. The cylinder oil is typically formulated to provide for good oxidation and thermal stability, water demulsibility, corrosion protection and good antifoam performance. Alkaline detergent additives are also present to neutralize acids formed during the combustion process. Disperaant, antioxidant, antifoam, antiwear and extreme pressure (EP) performance may also be provided by the use of suitable additives. Preferably the lubricating composition of the present invention comprises one or more additives selected from dispersants, detergents, antiwear agents, friction reducing agents, viscosity thickeners, metal passivators, acid sequestering agents, pour point depressants, corrosion inhibitors, defoaming agents, seal fix or seal compatibility agents and antioxidants. Preferably al 1 of the afore-mentioned listed additives are present in the lubricating oil composition of the present invention. Examples of such additives are for example described in US-B-6596673, v*hich publication is hereby incorporated by reference.

Detergents that may be conveniently used in the lubricating oil composition of the present invention, include one or more detergents selected from phenate detergents, salicylate detergents and sulphonate detergents. Alkali metal and alkaline earth metal salicylate, phenate and sulphonate detergents are preferred m the lubricating oil compositions of the present invention. Calcium and magnesium salicylates, phenates and sulphonates are particularly preferred detergents therein.

Detergents used in the lubricating oil composition of the present invention, may each, independently, have a TBN (Lotal base number) value in the range of from 30 to 350 mg KOH/g, preferably about 70 mg KOH/g, as measured by ISO 3771 and are preferably present in a total artioant in the range of irom 0.5 to 18 wt. %, based on the total weight of said lubricating oil composition.

The lubricating compositions herein preferably have a total base number (TBN) of aL least 10, more preferably in the range of from 10 to 60, even more preferably in thα range of from 10 to 50, and especially in the range of from 1C to 40 mg KOH/g, as measured by ISO 3771. Tn one preferred embodiment of the invention the TBK is in the range of from 10 to 15, preferably from 12 to 15. In another embodiment, the TBN is at least 20 mg KOH/g.

Antioxidants which may be conveniently used in the lubricating oil composition of the present invention, include one or more antioxidants selected from the group of aminic antioxidants and/or phenolic antioxidants. Said antioxidants may be generally present in a total amount in the range of from 0 to 2 wt. %, based on the total weight of said lubricating oil composition.

Examples of aminic antioxidants which may be conveniently used include alkylated diphenylamines, phenyl-α-naphthylamines, phenyl-β-naphthylamines and alkylated α-naphthylamines .

Preferred aminic antioxidants include dialkyldiohenylammes such as p,p' -dioctyl-diphenylamine, p,p' -di-ct-irethylbenzyl-diphenylamine and N-p-butylphenyl- N-ρ' -octylphenylamine, monoalkyldiphenylamines such as mono-t-butyldiphenylditiine and mono-octyldiohenylamine, bis (di al kylphenyl) amines such as di-~(2,4~ diethylphenyl) amine and di {2-ethyl~4-nonylphenyl) aπine, alkylphenyl-1-naphthylamir.es such as octylphenyl-l- naphthylamine and n-t-dodecylphenyl-1-naphthylamine, 1- naphthylamme, arylnaphthylamines such as phenyl-1™ naphthylamine, phenyl-2-naphthylamine _f N-hexylphenyl-2~ naph-hylamino and N-octylphenyl-2-naphthylamine, phenylenediaminea sαch as N, N' -diisopropyl-p- phenylenediamine and N, N' ~dipher.yl-p-phenylenediamine, and phenothiazines such as phenothi azine and 3,7- dioctylphenothiazine .

Preferred aminic antioxidants include those available under the following trade designations: "Sonoflex OD-3" {ex. Seiko Kagaku Co.), "Irganox L-57"

(ex. Ciba Specialty Chemicals Co.) and phenothiazinc (ex. IIodogaya Kagaku Co.) .

Examples of phenolic ant- oxidants which may be conveniently used include C ₇ -Cg branched alkyl esters of 3, 5-bis (1, 1-dimethyl-ethyl) -4-hydroxy-benzenepropanoic acid, 2-t~butylphenol, 2-t-butyl-4-methyl phenol, 2-t- buty] -5-τnethylphenol, 2, 4-di~t-butylphcnol, 2, 4-αimethyl- 6-t-butylphenol, 2-t-butyl-4-methoxypher.ol, 3-t~butyl~4- methoxyphenol, 2, 5-di-t-butylhydroquinone, 2,6-di-t- butyl-4-alkylpheno.l s such as 2, 6-di-t-butylphenol, 2,6- di-t-butyl-4-methylphenol and 2, 6-di-t-butyl-4- ethylphenol, 2, 6-di-t-butyl-4-al koxyphenols such as 2,6- di-t-butyl-4-methoxyphenol and 2, 6-di-t-butyl- 4-ethoxyphenoi, 3, 5~di-t-butyi~4- hydroxybenzylmercaptooctylacetate, alkyl-3- (3, 5-di-t- butyl-4-hydroxyphenyl) propionates such as n-octadecyl~3- (3, 5-di-t-butyl-4-hydroxyphenyl) propionate, n-butyl-3- (3, 5-di-t~butyl-4 -hydroxyphenyl) propionate and 2' - ethylhexyl-3- (3, 5~di-t-butyl-4-hydroxyphenyl) propionate, 2, 6-di-t-butyl-α-dimethylamino-p-cresol, 2, 2' - methylcne-bis (4-alkyl-6-t-butylphenol) such as 2,2 ^r ™ methylenebis (4-mcthyl-6-t-butylphenol, and 2,2- methylenebis (4-ethyl-6-t-butylphenol) , bisphenols such as 4,4' -butylidenebis (3-melhyl-6-t--butylphenol, 4,4'- methylenebis (2, 6-di-t-butylphenol) , 4, 4'-bi s (2, 6-di-t- butylphenol) , 2, 2- (di-p-hydroxyphenyl) propane, 2,2- bis (3, 5-di-t-butyl-4-hydroxyphenyl) propane, 4,4' -cyclohexylidenebis (2, 6-t-butylphenol) , hexamethylenegiycol-bis [3- (3, 5-di-t-butyl-4~ hydroxyphenyl) propionate] , triethyleneglycolbis___[3- (3-t- butyl-4-hydrox.y-5~methylphenyl) propionate] , 2,2' -thio- [diethyl-3- (3, 5-di-t-butyl-4-hyαroxyphenyl) propionate], 3, 9-bis{ 1, l-dimethyl-2- [3- {3-t-butyl-4- hydroxy-5-methylphenyl ) propionylcxy] ethyl } 2 , 4 , 8 , 10- tetraoxaspiro [5,5] undecane, 4,4' -thiobis (3-methyl-6-t- butylphenol) and 2, 2' -thiobis (4, 6-di---butyIresorcinol) , polyphenols such as tetrakis [methylene-3- (3, 5-di-t-butyl- 4 -hydroxyphenyl) propionate] methane, 1, 1, 3-tris (2-methyl- 4-hydroxy-5-t~buty] phenyl) butane, 1,3, 5-trimethyl-2, 4, 6- tris (3, 5-di-t-butyl-4-hydroxybenzyl) benzene, bis- [3, 3' - bis (4' ~hydroxy-3' -t-butylphenyl) butyri c acid] glycol ester, 2- {3' ,5' -di-t-butyl-4-hydroxyphenyl}methyl-4- (2",4"-di-t-butyl-3"-hydroxyphenyl)methyl-6-t-butylphenol and 2, 6-bis (2' -h.γdroxy-3' -t-butyl-5' -meLhylbenzyl) -A- methylphenol, and p-t-butylphenol - formaldehyde condensates and p-t-butylphenol - acetaldehyde condensates.

Preferred phenolic antioxidants include ~hose available under the following trade designations: "Irganox L-135" (ex. Ciba Specialty Chemicals Co.), "Anteeji DBH" (ex. Kawaguchi Kagaku Co.,), "Yoshinox SS" (ex. Yoshitomi Seiyaku Co.), "Antage W-400" (ex. Kawaguchi Kagaku Co.), "Antage W-500" (ex. Kawaguchi Kagaku Co.), "Antage W-300" (ex. Kawaguchi Kagaku Co.), "Ionox 220AH" (ex. Shell Japan Co.), bisphenol A, produced by the Shell Japan Co., "Irganox L109" (ex. Ciba Speciality Chemicals Co.), "Tominox 917" (ex. Yoshitomi Seiyaku Co. ) , "Irganox L115" (ex. Ciba Speciality Chemicals Co.), "Sumilizer GA80" {ex. Sumitomo Kagaku), "Antage RC" (ex. Kawaguchi Kagaku Co.), "Irganox LlOl" (ex. Ciba Speciality Chemicals Co.), "Yoshinox 930" (ex. Yoshitomi Seiyaku Co.), "Ionox 330" (ex. Shell Japan Co. ) .

In a preferred embodiment, the lubricating oil composition of the present invention may comprise one or more zinc dithiophosphates as antiwear additives, the or each zinc dithiophosphate being selected from zinc dialkyl-, diaryl-, or alkylaryl-ditni ophosphates . Zinc dialkyl dithiophosphates are particularly preferred.

Examples of suitable zinc dithiophosphates which are commercially available include those available ex.

Lubrizol Corporation under the trade designations "Lz 1097" and "Lz 1395", those available ex. Chevron Oronite under the trade designations "OLOA 26,7" and "OLOA 269R", and that available ex. Ξthyl under the trade designation "HITEC 7197"; zinc dithiophosphates such as those available ex. Lubrizol Corporation under the trade designations "Lz 677A", "Lz 1095" and "Lz 1371", uhat available ex. Chevron Oronite under the trade designation "OLOA 262" and -hat available ex. Ethyl under the trade designation "IIITEC 7169"; and zinc dithiophosphates such as those available ex. Lubrizol Corporation under the trade designations "Lz 1370" and "Lz 1373" and that available ex. Chevron Oronite under the trade designation "OLOA 260".

The lubricating oil composition according to the present invention may generally comprise in the range of from 0.1 to 1.5 wt. % of zinc diuhiophosphato, preferably in the range of from 0.4 to 0.9 wt. % and most preferably in the range of from 0.45 to 0.8 wt . %, based on total weight of the lubricating oil composition.

Further antiwear additives that may be conveniently used include molybdenum-containing compounds and boron- containing compounds.

Examples of such molybdenum-containing compounds may conveniently include molybdenum dithiocarbamates, trinuclear molybdenum compounds, for example as described in WO-A-98/26030, sulphides of molybdenum and molybdenum dithiophosphate.

Said molybdenum-containing antiwear additives may be conveniently added to the lubricating oil composition of the present invention in an amount in the range of from 0.1 to 3.0 wt. %, based on the total weight of lubricating oil composition.

Boron-containing compounds that may be conveniently used include borate esters, borated fatty amines, borated epoxides, alkali metal (or mixed alkali metal or alkaline earth metal) borates and borated overbased metal salts.

Said boron-containing anti-wear additives may be conveniently added to the lubricating oil composition of 5 the present invention in an amouni in the range of from 0.1 to 3.0 wt. %, based on the total weight of lubricating oil composition.

The lubricating oil compositions of the present invention may additionally contain one or more

10 dispersants which may be preferably admixed in an amount in the range of from 5 to 15 wt . %, based on the total weight of the lubricating oil composition.

Examples of dispersants which may be used include the polyalkenyl succiniinides and polyalkenyl sucσininic Ib acid esters disclosed in Japanese Patent Nos. 1367796, 1667140, 1302811 and 1743435. Preferred dispersants include borated succinimides .

Preferred friction reducing agents that may be conveniently used include fatty acid amides, more 20 preferably unsaturated fatty acid amides.

The total amount of friction reducing agents that may be added to the lubricating oil composition of the present invention is conveniently in the range of from 0.05 to 1.2 wt. %, based on the total weight of the 25 lubricating oil composition.

Polymethacrylates such as those as disclosed in Japanese Patent Nos. 1195542 and 1264056 may be conveniently employed in the lubricating oil compositions of the present invention as effective pour point 30 depressants.

Furthermore, compounds such as alkenyl succinic acid or ester moieties thereof, benzotriazole-based compounds and rhiodiazol e-based compounds may be conveniently used in the lubricating oil composition of the present invention as corrosion inhibitors.

Compounds such as polysiloxanes, dimethyl polycyclohβxane and polyacrylates may be conveniently used in the lubricating oil composition of the present invention as defoaming agents.

Compounds which may be conveniently used in the lubricating oil composition of the present invention as seal fix or seal compatibility agents include, for example, commercially available aromatic esters.

The lubricating oil compositions of the present invention may be conveniently prepared by admixing the base oil blend and the one or more additives selected from dispersants, detergents, antiwear agents, friction reducing agents, viscosity thickeners, metal passivators, acid sequestering agents, pour point depressants, corrosion inhibitors, defoaming agents, seal fix or seal compatibility agents and antioxidants.

In another embodiment of the present invention, there is provided a method of lubricating a marine or stationary low-speed crosshead diesel engine or a trunk piston medium speed diesel engine comprising applying a lubricating oil composition as hereinbefore described thereto. The present invention will now be described by reference to the following Examples which are not intended to limit the scope of the invention in any way. Examp]es 1 and 2 and Comparative Examples A - C

The lubricating compositions of the examples below were prepared by admixing the base oils and the additive package until homogeneous.

The compositions of the prepared lubricating compositions are set out in Table 1 below. Table 1

Comparative Example

Paraffinic API Group I base oil manufactured via the solvent extraction process having a kinematic viscosity at 300 'C of approximately 11 mmVs, commercially available from the Royal Dutch/Shell Group of Companies

Paraffinic API Group 1 base oil manufactured by the solvent extraction process having a kinematic viscosity at 100° C of approximately 32mm ^z /s cornroercially available from the Royal Dutch/Sholl Group of Companies

De-asphalted cylinder oil commercially available from the Royal Dutch/Shell Group of Companies having a kinematic viscosity at 100 "C of 66 mmVs, a density of 989 kg/m ^J and a pour point of 9 "C. Fischer-Tropsch derived Group 111 base oil with a kinematic viscosity at 100 "C of approximately 19 cΞt, a viscosity index of 142, a density of 837 kg/rn ³ and a pour point of -24 'C, which may conveniently be prepared by the process described in US Patent No. 7354508

Fischer-Tropsch derived Group III base oil with a kinematic v.iscosity at 100° C of approximately 8 mm ² /s, a viscosity index of 143-148, a density of 828 kg/m ³ and a pour point of -24 °C, which may conveniently be prepared by the process described in WO02/070631 6. Additive package comprising overbased detergent, zinc dithiophosphate as anti-wear agent, stabilisers, pour point depressant and anti-foair. agent Oxidation_S_tabilitγ Test Method

The Oxidation Stability of the lubricating compositions of Examples 1 to 3 and Comparative Example A was measured using the Oxidation Stability Test Method. The test method is carried out according to ASTM D6186. This test method makes use of Differential Scanning Calorimetry (DSC) for measuring the oxidation stability of a .lubricating composition. DSC is a technique for measuring the energy necessary to establish a near.ly zero tempera Lure difference between a substance and an inert reference material, as the two specimens are subjected zo identical temperature regimes in an environment heated or cooled at a controlled rate. Differences in temperature between the sample zone and the reference zone provide information about the thermodynamics (i.e. onset temperature and change in enthalpy) of the reaction. The differentia] aspect of the measurement allows only sma.1.1 differences to be amplified, eliminating large common™ mode signals, and greatly enhancing the sensitivity.

The Differential Scanning Calorimeter used in the Examples was supplied by TA Instrument. The tests were carried out at a temperature of 210° C and a pressure of 10 bar oxygen atmosphere. The results of the Oxidation Stability test are shown below in Table 2. Table 2

* Comparative Example

As can be seen from Table 2, the lubricating compositions of Examples 1 to 3 (containing a Fischer- Tropsch derived heavy base oil) have significantly improved oxidation stability, compared with the lubricating composition of Comparative Example A (based on conventional API Group I base oils) .