The invention relates to an oil composition comprising a an additive and a base oil component having a high viscosity index, a low sulphur content and a high saturates content . Base oils having a high viscosity index, a low sulphur content and a high saturates content are for example the so-called API Group III base oils of which the commercial XHVI base oil series as obtainable from Shell is an example. These base oils have a viscosity index (VI) of greater than 120, a sulphur content of below 0.03 wt% and a saturates content of greater than 90 wt%. Such base oils are typically prepared by hydroismersation of waxy feedstocks, for example petroleum derived slack wax, followed by a solvent or catalytic dewaxing treatment. Alternatively such base oils are prepared by catalytically dewaxing the distillation residue, bottoms, of the effluent of a fuels hydrocraker. Said API Group III base oils typically comprise substantially of a mixture of paraffins and naphthenic compound and a small content of aromatic and other polar compounds. The content of paraffins may vary but is typically below 70 wt%. It is also known that such base oils may be prepared from a Fischer-Tropsch wax as described in EP-A-776959 resulting in a base oil having a higher paraffin content. Although processes to prepare API Group III base oils from Fischer-Tropsch are known, only Shell actually prepares such base oils on a

commercial scale by solvent dewaxing a Waxy Raffinate product as obtained from Shell MDS Malaysia Sdn Bhd. WO-A-01/57166 discloses a liquid lubricant composition comprising a paraffinic biodegradable hydrocarbon basestock having a pour point of below -25 ⁰ C, and additives soluble in this feedstock. The document exemplifies a large number of different potential additives, among which figure a number of antioxidants, to be employed in an amount of from 0.1 wt% to 4 wt%. Oil compositions comprising base oils and an additive are used for many applications, for example in relatively highly additivated automotive crankcase engine lubricants and relatively lowly additivated industrial lubricant formulations such as hydraulic oils, compressor oils and steam or gas turbine oils. In said applications a high oxidative stability is required. The object of the present invention is to provide an oil composition with a high oxidative stability.

This object is achieved with the following oil composition. Oil composition comprising a base oil component having viscosity index of greater than 120, a sulphur content of below 0.03 wt%, and a saturates content of greater than 98 wt% and an additive, wherein the base oil component has a paraffin content of greater than 80 wt% and comprises a series of iso-paraffins having n, n+1, n+2, n+3 and n+4 carbon atoms and wherein n is between 20 and 40 and wherein the oil composition comprises more than 0.2 wt% of a hindered phenolic type anti-oxidant . Applicants found that when such a highly iso- paraffinic base oil is used in combination with the relatively high content of anti-oxidant additive a very oxidative stable oil compositions is obtained. It was

found that the so-called additive response of these base oils is much improved over the response with traditional API Group III base oils. The highly isoparaffinic base oil component according to the invention, and in particular Fischer-Tropsch derived base oils as base oil component show a synergistic and particularly non-linear response specifically to the presence of sterically hindered phenolic type anti-oxidants, in particular when the anti-oxidant is present in a range of from 0.2 wt% to 1.5 wt% in the base oil. This response is highly surprising, since Fischer-Tropsch derived base oils are usually more oxidatively stable than mineral oil derived base oils due to their purity and the absence of polar components therein. Other anti-oxidants such as aromatic amine antioxidants or non-phenolic oxidation inhibitors did not exhibit this behaviour.

The content of the sterically hindered phenolic type anti-oxidant is preferably greater than 0.4 wt%, yet more preferably greater than 0.5 wt%, again more preferably greater than 0.6 wt . Suitably the content of anti-oxidant additive is less than 5 wt%. Preferably, though, the upper limit for the content is preferably less than 2 wt%, more preferably less than 1.85 wt% since no particular improvement could be was visible above that level.

Any sterically hindered phenolic antioxidants may be used. Anti-oxidant additives of particular interest are selected from the group consisting of 2,6-di-tert- butylphenol (IRGANOX TM L 140, CIBA), BHT, 2 , 2 ' -methylene bis- (4, 6-di-tert-butylphenol) , 1, 6-hexamethylene-bis- (3, 5-di-tert-butyl-hydroxyhydrocinnamate) (IRGANOX TM L109, CIBA), ( (3, 5-bis (1,1-dimethylethyl) -4- hydroxyphenyl) methyl) thio) acetic acid, C _] _o~Ci4isoalkyl

esters (IRGANOX TM LIl8, CIBA), 3 , 5-di-tert-butyl-4- hydroxyhydrocinnamic acid, C ₇ -C9alkyl esters (IRGANOX TM

L135, CIBA,) tetrakis- (3- (3 , 5-di-tert-butyl-4- hydroxypheny1) -propionyloxymethyl) methane (IRGANOX TM 1010, CIBA), thiodiethylene bis (3 , 5-di-tert-butyl-4- hydroxyhydrocinnaraate (IRGANOX TM 1035, CIBA) , octadecyl 3, 5-di-tert-butyl-4-hydroxyhydrocinnamate (IRGANOX TM 1076, CIBA) and 2 , 5-di-tert-butylhydroquinone . These products are known and are commercially available. Of most particular interest is 3 , 5-di-tert-butyl-4-hydroxy- hydrocinnamic acid-CY-Cg-alkyl ester. In the above list reference is made to CIBA. With CIBA is meant CIBA Ltd

Basel Switzerland.

Preferred sterically hindered phenolic type anti-oxidants include 4 , 4 -methylene bis-2 , 6-ditertiarybutyl phenol, 3- t-butyl-4-methoxyphenol, 2 , 5-di-t-butylhydroquinone, n-octadecyl-3- (3 , 5-di-t-butyl-4-hydroxyphenyl) - proprionate, 2 , 2' -methylenebis (4-methyl-6-t-butylphenol) , 2,2' -methylenebis (3-methyl-6-t-butylphenol) , 4 , 4 ' -butylidenebis (4-ethyl-6-t-butylphenol) ,

4,4' -methylenebis (2 , 6-t-butylphenol) , 2,2- (di-p- hydroxyphenyl) propane , hexamethyleneglycol bis [3- (3 , 5-di- t-butyl-4-hydroxyphenyl)priopionate] , triethyleneglycol bis [3- (3-t-butyl-4-hydroxy-5-methylphenyl) priopionate] , 2,2' -thio- [diethyl-3- (3, 5-di-t-butyl-4-hydroxyphenyl) - priopionate], or 3 , 9-bis{l, l-dimethyl-2- [3- (3-t-butyl-4- hydroxy-5-methylphenyl)proprionyloxy] -ethyl} -2, 4, 8, 10- tetraoxospiro [5,5] undecane .

The base oil has a viscosity index of greater than 120, preferably a VI of greater than 130. The kinematic viscosity at 100 ⁰ C of the base oil may range from 2 to

25 mm ² /sec. Low viscosity base oils for this invention are those having a viscosity at 100 ⁰ C of between 2 and

4 mm ² /sc. Medium viscosity base oils have a kinematic viscosity at 100 ⁰ C of between 4 and 7 mm ² /sec. Medium heavy viscosity grade base oils have a kinematic viscosity at 100 ⁰ C of between 7 and 12 mm ² /sec and high viscosity base oils have a kinematic viscosity at 100 ⁰ C of between 12 and 25 mm ² /sec. The upper limit will be dependent on the viscosity of the base oil and may range up to 170 for the high viscosity type base oils. The sulphur content is below 0.03 wt%, preferably below 100 ppm and even more preferably below 10 ppm. The saturates content is greater than 98 wt%, preferably greater than 99 wt . The pour point will depend partly on the severity of the optional dewaxing process used to prepare said base oils and partly on the viscosity of the base oils. The low viscosity base oils typically have a lower pour point than the higher viscosity grade base oils. The pour point may therefore range from values of +10 ⁰ C for the high viscosity grade base oils to -60 ⁰ C for the low viscosity grade base oils. The base oil comprises a series of iso-paraffins having n, n+1, n+2, n+3 and n+4 carbon atoms and wherein n is between 20 and 40 and the paraffin content in the base oil is greater than 80 wt%, preferably greater than 90 wt%. The main other component in the highly saturated base oil are suitably naphthenic compounds. The content of paraffinic compounds and the presence of such a continuous series of iso-paraffins may be measured by Field desorption/Field Ionisation (FD/FI) mass spectrometry technique. In this technique the oil sample is first separated into a polar (aromatic) phase and a non-polar (saturates) phase by making use of a preparative high performance liquid chromatography (HPLC) method IP368/01, wherein as mobile phase pentane is used

instead of hexane as the method states. The saturates and aromatic fractions are then analyzed using a Finnigan MAT90 mass spectrometer equipped with a Field desorption/Field Ionisation (FD/FI) interface, wherein FI (a "soft" ionisation technique) is used for the determination of hydrocarbon types in terms of carbon number and hydrogen deficiency. The type classification of compounds in mass spectrometry is determined by the characteristic ions formed and is normally classified by "z number". This is given by the general formula for all hydrocarbon species: C _n H2n+z- Because the saturates phase is analysed separately from the aromatic phase it is possible to determine the content of the different iso- paraffins having the same stoichiometry or n-number. The results of the mass spectrometer are processed using commercial software (poly 32; available from Sierra Analytics LLC, 3453 Dragoo Park Drive, Modesto, California GA95350 USA) to determine the relative proportions of each hydrocarbon type. The above described base oil are preferably obtained by hydroisomerisation of a paraffinic wax, preferably followed by some type of dewaxing, such as solvent or catalytic dewaxing. The paraffinic wax may be a highly paraffinic slack wax. More preferably the paraffinic wax is a Fischer-Tropsch derived wax, because of its purity and even higher paraffinic content. The base oils as derived from a Fischer-Tropsch wax as here described will be referred to in this description as Fischer-Tropsch derived base oils . Examples of Fischer-Tropsch processes which for example can be used to prepare the above-described Fischer-Tropsch derived base oil are the so-called commercial Slurry Phase Distillate technology of Sasol,

the Shell Middle Distillate Synthesis Process and the "AGC-21" ExxonMobil process. These and other processes are for example described in more detail in EP-A-776959, EP-A-668342, US-A-4943672 , US-A-5059299 , WO-A-9934917 and WO-A-9920720. Typically these Fischer-Tropsch synthesis products will comprise hydrocarbons having 1 to 100 and even more than 100 carbon atoms. This hydrocarbon product will comprise normal paraffins, iso-paraffins, oxygenated products and unsaturated products. If base oils are one of the desired iso-paraffinic products it may be advantageous to use a relatively heavy Fischer-Tropsch derived feed. The relatively heavy Fischer-Tropsch derived feed has at least 30 wt%, preferably at least 50 wt%, and more preferably at least 55 wt% of compounds having at least 30 carbon atoms. Furthermore the weight ratio of compounds having at least 60 or more carbon atoms and compounds having at least 30 carbon atoms of the Fischer-Tropsch derived feed is preferably at least 0.2, more preferably at least 0.4 and most preferably at least 0.55. Preferably the Fischer-Tropsch derived feed comprises a C20+ fraction having an ASF-alpha value

(Anderson-Schulz-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. Such a Fischer-Tropsch derived feed can be obtained by any process, which yields a relatively heavy Fischer-Tropsch product as described above. Not all Fischer-Tropsch processes yield such a heavy product. An example of a suitable Fischer-Tropsch process is described in WO-A-9934917.

The Fischer-Tropsch derived product will contain no or very little sulphur and nitrogen containing compounds. This is typical for a product derived from a Fischer-

Tropsch reaction, which uses synthesis gas containing almost no impurities. Sulphur and nitrogen levels will generally be below the detection limits, which are currently 5 mg/kg for sulphur and 1 mg/kg for nitrogen respectively.

The process will generally comprise a Fischer-Tropsch synthesis, a hydroisomerisation step and an optional pour point reducing step, wherein said hydroisomerisation step and optional pour point reducing step are performed as: (a) hydrocracking/hydroisomerisating a Fischer-Tropsch product,

(b) separating the product of step (a) into at least one or more distillate fuel fractions and a base oil or base oil intermediate fraction. If the viscosity and pour point of the base oil as obtained in step (b) is as desired no further processing is necessary and the oil can be used as the base oil according the invention. If required, the pour point of the base oil intermediate fraction is suitably further reduced in a step (c) by means of solvent or preferably catalytic dewaxing of the oil obtained in step (b) to obtain oil having the preferred low pour point. The desired viscosity of the base oil may be obtained by isolating by means of distillation from the intermediate base oil fraction or from the dewaxed oil the a suitable boiling range product corresponding with the desired viscosity. Distillation may be suitably a vacuum distillation step.

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 skilled 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/ hydroisomerisation 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 hydroconversion/ hydroisomerisation catalysts are those comprising at least one Group VIB metal, preferably tungsten and/or molybdenum, and at least one non-noble Group VIII metal, preferably nickel and/or cobalt, as the hydrogenation component. Both metals may be present as oxides,

sulphides or a combination thereof . The Group VIB 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. A hydroconversion catalyst of this type, which has been found particularly suitable, is a catalyst comprising nickel and tungsten supported on fluorided 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. Preferably at 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- sulphided 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 hydrogenolysis 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 SiC>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 IVA 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. W., and Wilson, J.N., Cracking Catalysts, Catalysis: volume VII, Ed. Paul H. Emmett, Reinhold Publishing Corporation, New York, 1960, pp. 5-9.

The catalyst is prepared by co-impregnating the metals from solutions onto the support, drying at

100-150 ⁰ C, and calcining in air at 200-550 ⁰ C. The Group VIII metal is present in amounts of about 15 wt% or less, preferably 1-12 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:

Ni, wt% 2.5-3.5

Cu, wt% 0.25-0.35

Al ₂ O ₃ -SiO ₂ wt% 65-75 Al ₂ O ₃ (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 those based on molecular sieve type materials, suitably comprising at least one Group VIII metal component, preferably Pt and/or Pd, as the hydrogenation component. Suitable zeolitic and other aluminosilicate materials, then, include Zeolite beta, Zeolite Y, Ultra Stable Y, ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48, MCM-68, ZSM-35, SSZ-32, ferrierite, mordenite and

silica-aluminophosphates, such as SAPO-Il and SAPO-31. Examples of suitable hydroisomerisation/ hydroisoraerisation 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, ZSM-22, ZSM-23, ZSM-48, MCM-68, ZSM-35, SSZ-32, ferrierite, 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-20040065581 and US-A-20040065588.

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 oil to be used in the present invention. More preferred the first and second hydroisomerisation steps are performed in series flow. Most preferred the two steps are performed in a single reactor comprising beds of the above amorphous and/or crystalline catalyst.

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 370 ⁰ C which reacts per pass to a fraction boiling below 370 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) the product of step (a) is preferably separated into one or more distillate fuels fractions and a base oil or base oil precursor fraction having the desired viscosity properties. If the pour point is not in the desired range the pour point of the base oil is further reduced by means of a dewaxing step (c) , preferably by catalytic dewaxing. In such an embodiment it may be a further advantage to dewax a wider boiling fraction of the product of step (a) . From the resulting dewaxed product the base oil and oils having a desired viscosity can then be advantageously isolated by means of distillation. Dewaxing is preferably performed by catalytic dewaxing as for example described in WO-A-02070629, p. 10 line 23 to p. 14, line 2, which publication is hereby incorporated by reference. The final boiling point of the feed to the dewaxing step (c) may be the final boiling point of the product of step (a) or lower if required.

The oil composition according to the present invention may find use as a component of a relatively highly additivated automotive crankcase engine lubricant and relatively lowly additivated industrial lubricant formulation such as a hydraulic oil, a compressor oils and steam or gas turbine oil or combined steam/ gas turbine oil.

The invention will be illustrated by the following non- limiting examples. Example 1

To four base oils having the properties as listed in Table 1 an increasing quantity of AN2 (4 , 4 -methylene bis- 2 , 6-ditertiarybutyl phenol) additive was added (see Table 1) . The induction period at 180 ⁰ C and 200 psig oxygen (no flow) was measured according to

ASTM D 6186-88. This method determines the oxidation induction time of lubricating oils subjected to oxygen by pressure differential scanning calorimetry (PDSC) . The results are presented in Table 1.

Table 1

Table 1 (cont'd)

(*) Saturates content is 100% minus the polar content

Base Oil A was obtained by catalytically dewaxing a partly hydroisomerised waxy raffinate according to the procedures described in the examples of WO-A-02/070629.

Base Oil B and D were obtained by solvent dewaxing a hydrocrackate of a petroleum derived slack wax.

Base Oil C was obtained by catalytic dewaxing of a fuels hydrocracker bottoms (residue) .

The Base Oil A clearly shows a non- linear response to the addition of (4 , 4 -methylene bis-2 , 6-ditertiarybutyl phenol) in an amount above 0,4 wt%.