Such lubricant compositions are also referred to as SAE xW-y compositions. SAE stands for Society of Automotive Engineers in the USA. The"x"number in such a designation is associated with a maximum viscosity requirement at low temperature for that composition as measured typically by a cold cranking simulator (VdCCS) under high shear. The second number"y"is associated with a kinematic viscosity requirement at 100 °C as indicated below:

Engine Oil Viscosity Grade Specification (SEA J300) Low temperature High-temperature viscosities Viscosities SAE CCS MRV Kinematic HTHS Grade Viscosity Viscosity viscosity at Viscosity (cP) (cP) 100 °C (cSt) (cP) Min. Max. 0 W 3250 at 60000 at 3.8 - 30 °C-40 °C 5 W 3500 at 60000 at 3.8 - 25 °C-40 °C 10 W 3500 at 60000 at 4. 1 - 20 °C-40 °C 15 W 3500 at 60000 at 5.6 - 15 °C-40 °C 20 W 4500 at 60000 at 5. 6 - 10 °C-40 °C 25 W 6000 at 60000 at 9. 3 - 5 °C-40 °C 20 5. 6 < 9. 3 2. 6 min 30 9. 3 < 12.5 2.9 min 40 12. 5 < 16.3 2.9 min (PCEO) 40'12. 5 < 16.3 3.7 min (CEO) 50 16. 3 < 21.9 3.7 min 60 21. 9 < 26.1 3. 7 min A lubricant composition comprising a Fischer-Tropsch derived base oils and one or more additives is for example described WO-A-0157166. According to this publication the lubricant may comprise a so-called viscosity modifier polymer. Such an additive is typically a relatively high molecular weight component, which has a marked viscosity thickening property when blended with

the other components and the base oil. Such high molecular weight materials are generally polymeric materials, known alternatively as viscosity modifier polymers, polymeric thickeners, or viscosity index improvers.

WO-A-0157166 discloses lubricant formulations not containing viscosity modifier polymers. Especially lubricant formulations containing no viscosity modifier polymer may conform to the SAE"xW-y"viscosity grading, where x = 0,5, 10, or 15, and where = 10,20, 30 or 40, and where (y-x) is less than or equal to 25.

According to the same publication it has been found that in many cases, viscosity modifier polymers in combination with lower viscosity basestocks have been found to be highly advantageous in achieving desired viscometric targets, particularly with multigrade lubricant oils. Furthermore it is disclosed that more widely cross-graded lubricant formulations such as the OW-40, 5W-50 and 10W-60 will normally require more of the high molecular weight polymer thickener than less widely cross-graded lubricant formulations, for example OW-20 and 10W-30 oils which will need little or none of this thickening material.

The examples 11-14 of WO-A-0157166 show that non- viscosity modified lubricant formulation according to SAE OW-20, SAE 5W-20 and SAE 10W-30 can be obtained by blending a poly-alpha olefin and a Fischer-Tropsch derived base oil, wherein the FT-derived base oil had a kinematic viscosity at 100 °C of respectively 3.7, 4.0, 4.1 and 6.0 cSt. According to this specification VISCOSITY MODIFIER-free lubricant formulations have been only achieved in a formulation also comprising poly-alpha olefins.

Applicants found that when a lubricant consisting of a viscosity modifier as referred to above is used as

motor engine lubricant in gasoline direct injection (GDI) engines a residues on the back of the inlet valve tulip tend to build up. This is very disadvantageous because these residues tend to harden and break off from the surface where they may enter the motor engine cylinder, which may in turn cause the engine to failure.

It is an object of the present invention to provide a lubricant formulation which, when used in a GDI engine, does not result in a build up of residues.

This object is achieved with the following lubricant composition. A lubricant composition comprising a mixture of at least two Fischer-Tropsch derived base oils and one or more additives wherein one Fischer-Tropsch derived base oil has a kinematic viscosity at 100 °C of less than 7 cSt and the second Fischer-Tropsch derived base oil has a kinematic viscosity at 100 °C of more than 18 cSt.

Applicants found that by blending the relatively high viscosity Fischer-Tropsch derived base oil with the lower viscosity grade base oil it is possible to achieve the properties of a SAE"xW-y"viscosity lubricant formulation without having to add a viscosity modifier.

Applicants further found that such a VISCOSITY MODIFIER- free lubricant may be obtained without having to add a poly-alpha olefin co-base oil as shown in WO-A-0157166.

Thus a VISCOSITY MODIFIER free lubricant based totally on a Fischer-Tropsch derived base oils may be advantageously be formulated. Applicants further found that when a viscosity modifier-free lubricant is used as motor engine lubricant in gasoline direct injection (GDI) engines no build up of residue on the back of the inlet valve tulip occurs.

It has further been found that especially SAE"xW-y" viscosity lubricant formulations wherein y-x is greater or equal than 25 can be prepared without having to add a VISCOSITY MODIFIER. Based on the teaching of WO-A-0157166

one would have expected that such formulations could only be prepared by having to add a VISCOSITY MODIFIER.

The Fischer-Tropsch derived base oil having a kinematic viscosity at 100 °C of less than 7 cSt (also referred to as the"low viscosity component") preferably has a pour point of less than-18 °C, more preferably less than-30 °C. The kinematic viscosity at 100 °C is preferably greater than 3.5 cSt and more preferably between 3.5 and 6 cSt. The viscosity index (VI) is preferably greater than 120, more preferably greater than 130. The VI will typically be less than 160. The Noack volatility (according to CEC L40 T87) is preferably less than 14 wt%. The low viscosity component may any Fischer- Tropsch derived base oil as disclosed in for example EP-A-776959, EP-A-668342, WO-A-9721788, WO-0015736, WO-0014188, WO-0014187, WO-0014183, WO-0014179, WO-0008115, WO-9941332, EP-1029029, WO-0118156 and WO-0157166.

The second Fischer-Tropsch derived base oil, having a kinematic viscosity at 100 °C of more than 18 cSt, will also be referred to as the"high viscosity component".

Preferably the high viscosity component has a-kinematic viscosity at 100 °C of between 20 and 40 cSt and more preferably between 20 and 30 cSt. The pour point may suitably range between-50 and +20 °C. It has been found that the pour point of the heavy base oil is less critical and that even base oils having a pour point of above 0 °C do not negatively affect the low temperature properties of the final lubricant formulation. The viscosity index is preferably higher than 150 and more preferably between 160 and 190. It is believed that this class of Fischer-Tropsch derived base oils is new.

The volume ratio of the low and high viscosity components in the final base oil can be determined by making use of well known blending rules which are based

on the properties of the starting components and the desired SAE xW-y lubricant formulation one intends to arrive at. The lubricant composition suitably comprises between 65 and 95 wt% of the Fischer-Tropsch derived base oils. The remaining part of the composition consists of one or more additives. Optionally part of the lubricant composition may comprise of a second base oil, for example PAO, petroleum derived based base oil or esters.

This fraction will suitably be less than 10 wt%. The advantages of the invention are however fully appreciated, as explained above, when only the Fischer- Tropsch derived base oils are used as base oil according to the present invention or at least when no significant volumes of the more expensive poly-alpha olefins are present.

The invention is also directed to the general use of the heavy grade base oil as described above in motor oil formulations, which do not require a viscosity modifier.

The heavy base oil may be combined with another Fischer- Tropsch derived base oil to formulate the above lubricant formulations or in combination with other base oils.

Other base oils are for example polyalphaolefins, esters, polyalkylenes, alkylated and aromatics, and more preferably mineral oils, for example hydrocrackates and solvent-refined basestocks.

The lubricant formulation according to the present invention preferably does not comprise a VISCOSITY MODIFIER. The lubricant formulation may comprise one or more other additives as for example described in the afore mentioned WO-A-0157166. Examples of additive types which may form part of the composition are dispersants, detergents, extreme pressure/antiwear additives, antioxidants, pour point depressants, emulsifiers, demulsifiers, corrosion inhibitors, rust inhibitors, antistaining additives and friction modifiers. Specific

examples of such additives are described in for example Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526.

Suitably the anti-wear additive is a zinc dialkyl dithiophosphate. Suitably the dispersant is an ashless dispersant, for example polybutylene succinimide polyamines or Mannic base type dispersants : Suitably the detergent is an over-based metallic detergent, for example the phosphonate, sulfonate, phenolate or salicylate types as described in the above referred-to General Textbook. Suitably the antioxidant is a hindered phenolic or aminic compound, for example alkylated or styrenated diphenylamines or ionol derived hindered phenols. Examples of suitable antifoaming agents are polydimethylsiloxanes and polyethylene glycol ethers and esters.

A preferred process that is capable of preparing both the low viscosity base oil component and the high viscosity base oil component is described below.

This process comprises a hydrocracking/hydro- isomerisation step on a relatively heavy feed as obtained in a Fischer-Tropsch synthesis step. The fraction of the effluent of said hydroprocessing step, which base oil precursor fraction boils in the base oil range, is subsequently subjected to a dewaxing step. From the dewaxed effluent the low and heavy base oil component is subsequently isolated.

The relatively heavy feed to the hydrocracking/ 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 least 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-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. The initial boiling point of the feed is preferably below 200 °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 WO-A-9934917 and in AU-A-698392.

The hydrocracking/hydroisomerisation step is suitably performed in the presence of hydrogen and a catalyst, known to one skilled in the art as being suitable for this reaction. Examples of such processes are disclosed in WO-A-0014179, EP-A-532118, EP-A-666894 and EP-A-776959. A typical catalyst useful for this step is for example a 0.8 wt% platinum (Pt) loaded amorphous silica-alumina catalyst having a surface area of 392 m2/g and a pore volume, measured by mercury porosimeter, of 0.59 ml/g. Another catalyst useful for this hydroprocessing step is a amorphous silica alumina catalyst comprising between 2.5 and 3.5 wt% nickel (Ni), between 0.25 and 0.35 wt% Copper (Cu), between 65 and 75 wt% of amorphous silica-alumina, between 25 and 35 wt% alumina binder and wherein the final catalyst has a surface area of between 290 and 325 m2/g, a total pore volume (Hg) of between 0.35 and 0.45 ml/g and a compacted bulk density of between 0.58 and 0.68 g/ml.

The hydrocracking/hydroisomerisation step is preferably performed at a reaction temperature of between 175 to 380 °C, preferably higher than 250 °C and more

preferably from 300 to 370 °C. The pressure will typically be in the range of from 10 to 250 bara and preferably between 20 and 80 bara. 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 the hydrocracking/ hydroisomerisation step as defined as the weight percentage of the feed boiling above 370 °C which reacts per pass to a fraction boiling below 370 °C, is at least 20 wt%, preferably at least 25 wt%, but preferably not more than 80 wt%, more preferably not more than 70 wt%.

The feed as used above in the definition is the total hydrocarbon feed, including for example any recycle streams.

From the effluent of the hydrocracking/ hydroisomerisation step a high boiling fraction is isolated by means of distillation, which fraction boils in the base oil range.

Suitably two base oil precursor fractions are isolated, which fractions have a boiling range corresponding to the low and high viscosity components.

The precursor fractions are obtained in a vacuum distillation of the above referred to high boiling fraction. This base oil precursor fraction will suitably have an initial boiling point of between 330 and 400 °C.

The separation is preferably performed by means of a distillation at about atmospheric conditions, preferably at a pressure of between 1.2-2 bara, wherein next to said base oil precursor fraction suitably also a gas oil, naphtha and/or kerosine fractions are isolated.

The base oil precursor fraction is subsequently subjected to a dewaxing step (also referred to as a pour point reducing treatment). The dewaxing step can be performed by means of a so-called solvent dewaxing process hydrocracking/hydroisomerisation step as for example described in Lubricant Base Oil and Wax Processing, Avilino Sequeira, Jr, Marcel Dekker Inc. , New York, 1994, Chapter 7. But preferably the dewaxing step is performed by means of a catalytic dewaxing step.

Catalytic dewaxing is well known to the skilled reader and is suitably 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 VIII 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 nm. 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-11 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-11.

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 °C, suitably from 250 to 400 °C, hydrogen pressures in the range of 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/1/hr, more suitably from 0.5 to 3 kg/l/hr and hydrogen to oil ratios in the range of from 100 to 2,000 litres of hydrogen per litre of oil. By varying the temperature between 275 and more preferably between 315 and 375 °C at between 40-70 bars, in the catalytic dewaxing step it is possible to prepare the low viscosity base oil component having pour point values varying suitably from below-60 up to-10 °C.

From the effluent of the dewaxing step lower boiling non-base oil fractions are suitably first removed, preferably by means of distillation, optionally in combination with an initial flashing step. After removal of these lower boiling compounds the dewaxed product is separated, suitably by means of distillation, into at least the low and high viscosity base oil components. The high viscosity base oil component will suitably be the bottom product (or in other words: the highest boiling fraction) of such a distillation.

The invention will be illustrated with the following non-limiting example.

Example 1 The C5-C750 °C+ fraction of the Fischer-Tropsch product, as obtained in Example VII using the catalyst of Example III of WO-A-9934917, was continuously fed to a hydrocracking step. The feed contained about 60 wt% C30+ product. The ratio C60+/C30+ was about 0.55. In the hydrocracking step the fraction was contacted with a hydrocracking catalyst of Example 1 of EP-A-532118. The

effluent of step (a) was continuously distilled under vacuum to give lights, fuels and a residue"R"boiling from 370 °C and above. The yield of gas oil fraction on fresh feed to hydrocracking step was 43 wt%. The properties of the gas oil thus obtained are presented in Table 3.

The main part of the residue"R"was recycled to step and a remaining part was sent to a catalytic dewaxing step. The conditions in the hydrocracking step were: a fresh feed Weight Hourly Space Velocity (WHSV) of 0.8 kg/l. h, recycle feed WHSV of 0.25 kg/l. h, hydrogen gas rate = 1000 Nl/kg, total pressure = 40 bar, and a reactor temperature of 335 °C.

In the dewaxing step, the fraction described above boiling from 370 °C to above 750 °C was contacted with a dealuminated silica bound ZSM-5 catalyst comprising 0. 7% by weight Pt and 30 wt% ZSM-5 as described in Example 9 of WO-A-0029511. The dewaxing conditions were 40 bar hydrogen, WHSV = 1 kg/l. h and a temperature of 365 °C.

The dewaxed oil was distilled into three base oil fractions boiling between 305 and 420 °C (yield based on feed to dewaxing step was 16.1 wt%), between 420-510 °C (yield based on feed to dewaxing step was 16.1 wt%) and a fraction boiling above 510 °C (yield based on feed to dewaxing step was 27.9 wt%). The base oil fraction boiling between 420 and 510 °C and the heavier fraction was analysed in more detail (see Table 1).

Table 1 Low High viscosity viscosity component component density at 20 °C 818. 5 837. 0 pour point (°C)-59 +9 kinematic viscosity at 40 °C (cSt) 24.5 kinematic viscosity at 100 °C (cSt) 4. 9 22. 92 VI 128 178 sulphur content (% w) < 0. 001 < 0. 001 saturates (% w) > 95 > 95 Preparation of the dewaxing catalyst for use in Examples 2-3.

MTW Type zeolite crystallites were prepared as described in"Verified synthesis of zeolitic materials"as published in Micropores and mesopores materials, volume 22 (1998), pages 644-645 using tetra ethyl ammonium bromide as the template. The Scanning Electron Microscope (SEM) visually observed particle size showed ZSM-12 particles of between 1 and 10 um. The average crystallite size as determined by XRD line broadening technique was 0.05 um. The crystallites thus obtained were extruded with a silica binder (10% by weight of zeolite, 90% by weight of silica binder). The extrudates were dried at 120 °C. A solution of (NH4) 2SiF6 (45 ml of 0.019 N solution per gram of zeolite crystallites) was poured onto the extrudates. The mixture was then heated at 100 °C under reflux for 17 h with gentle stirring above the extrudates. After filtration, the extrudates were washed twice with deionised water, dried for 2 hours at 120 °C and then calcined for 2 hours at 480 °C.

The thus obtained extrudates were impregnated with an aqueous solution of platinum tetramine hydroxide followed by drying (2 hours at 120 °C) and calcining (2 hours at 300 °C).

The catalyst was activated by reduction of the platinum under a hydrogen rate of 100 1/hr at a temperature of 350 °C for 2 hours. The resulting catalyst comprised 0.35% by weight Pt supported on the dealuminated, silica-bound MTW zeolite.

Example 2 A partly isomerized Fischer-Tropsch derived wax having the properties as in Table 2 was distilled into a light base oil precursor fraction boiling substantially between 390 and 520 °C and a heavy base oil precursor fraction boiling above 520 °C.

Table 2 Density at 70 °C (kg/1) 0.7874 TlOwt% (°C) 346 T50wt% (°C) 482 T90wt% (°C) 665 Wax congealing point (°C) 48 The heavy base oil precursor fraction was contacted with the above-described dewaxing catalyst. The dewaxing conditions were 40 bar hydrogen, WHSV = 1 kg/l. h, a temperature of 340 °C and a hydrogen gas rate of 700 Nl/kg feed.

The dewaxed oil was distilled into two base oil fractions having the properties listed in Table 3.

Table 3 Light base oil Heavy base oil Boiling range of 390-520 >520 base oil product (°C) Yield on feed to 6. 2 54. 3 dewaxer Density at 20 °C 0. 8144 0. 8336 (kg/1) Pour point (°C) Not measured-42 Kinematic viscosity 4.339 15.95 at 100 °C (cSt) Viscosity Index 136 168 Average molecular 403 692 weight

The light base oil precursor fraction was also catalytically dewaxed by contacting with the above described dewaxing catalyst. The dewaxing conditions were 40 bar hydrogen, WHSV = 1 kg/l. h, a temperature of 310 °C and a hydrogen gas rate of 700 Nl/kg feed.

The dewaxed oil was distilled into two base oil fractions having the properties listed in Table 4.

Table 4 Targeted oil grade Base oil grade-4 Base oil grade-5.5 Targeted Boiling 400-455 420-520 range of base oil product (°C) Yield on feed to 33. 7% 63.3% dewaxer Density at 20 °C 0. 8124 0. 8183 (kg/1) Pour point (°C)-32-22 Kinematic viscosity 4.00 5.537 at 100 °C (cSt) Viscosity Index 132 144 Average molecular 385 451 weight T (10%), (50%), (90%) 397/430/456 417/462/522 from TBP-GLC Above, the distillation of the effluents of the dewaxing of the heavy and light base oil precursor fractions was done separately. It will be clear to the skilled person that the said effluents can also be combined before distillation into the various base oil products.

Example 3 Example 2 was repeated starting party isomerized Fischer- Tropsch derived wax having the properties as listed in Table 5. This feed was distilled into a light base oil precursor fraction boiling substantially between 390 and

520 °C and a heavy base oil precursor fraction boiling above 520 °C.

Table 5 Tl0wt% (°C) 549 T50wt% (°C) 656 T90wt% (°C) > 750 Congealing Point (°C) +106 Viscosity Vk at 150 °C 15. 07 cSt The heavy base oil precursor fraction was contacted with the above-described dewaxing catalyst. The dewaxing conditions were 40 bar hydrogen, WHSV = 1 kg/l. h, a temperature of 355°C and a hydrogen gas rate of 700 Nl/kg feed.

The dewaxed oil was distilled into two base oil fractions having the properties listed in Table 6.

Table 6 Light base oil Heavy base oil Boiling range of 390-520 >520 base oil product (OC) Yield on heavy feed 7. 7 47 to dewaxer Density at 20 °C 0. 8191 0. 829 (kg/1) Pour point (°C) Not measured-15 Kinematic viscosity 5.315 26.65 at 100 °C (cSt) Viscosity Index 132 157 Average molecular 435 788 weight Example 4 This example illustrates the use of a heavy Fischer- Tropsch derived base oil grade as part of a 5W-30

lubricant composition according to the so-called SAE J300 classification without having to use a viscosity modifier. The properties of the Fischer-Tropsch derived base oils and the resulting lubricant are presented in Table 7.

Table 7 Light Heavy 5W-30 Specification base base oil lubricant for a 5W-30 oil formulation lubricant according to SAE J-300 Light base 100% 68.8 oil Additive 10 Package (*) Pour point 0.2 depressant Heavy base 100% 21 oil Analysis MRV @ 13, 415 and < 60,000 - 35 °C, 13,475 centi Poise Yield no yield no yield stress stress @ stress - 35 °C Vdccs @ 18. 74 64. 11 66. 00 max. - 30 °C, Poise Kinematic 4. 979 9. 517 9. 3 to 12.5 viscosity at 100 °C (cSt) 24.53 Kinematic 25. 22 viscosity at 40 °C (cSt) PourPoint-54-51 (°C) +12 (*) the additive package was a standard package not containing a viscosity modifier additive.