Field of the invention The invention is related to a process to prepare a base oil having a kinematic viscosity at 100 0C of greater than 7 cSt from a Fischer-Tropsch derived wax by performing first a hydro-isomerisation followed by a dewaxing step. Background art Such a process is described in EP-A-776959. This publication discloses that a base oil having a kinematic viscosity at 100 0C of about 5 cSt can be obtained by subjecting a Fischer-Tropsch derived wax having a congealing point of about 70 0C and a T10wt% recovery point of about 430 0C to a hydro-isomerisation step followed by a solvent or catalytic dewaxing step. The yield of base oil was reported to be about 40 wt% when performing the process by means of solvent dewaxing. A disadvantage of the process as disclosed in EP-A-776959 is that when base oils having a higher viscosity are desired significantly lower yields are found. The object of the present invention is thus to improve the yield of base oils having a kinematic viscosity of greater than 7, especially greater than 8 cSt at 100 0C. Summary of the invention This object is achieved with the following process. Process to prepare a base oil having a kinematic viscosity at 100 0C of greater than 6 cSt from a Fischer- Tropsch derived wax having a T10wt% recovery boiling point of above 500 °C by performing the following steps, (a) contacting a feed comprising the Fischer-Tropsch wax and between 5 and 40 wt% of a hydrocarbon diluent having a T90wt% recovery point of below 400 0C with a hydro-isomerisation catalyst under hydro-isomerisation conditions, (b) dewaxing the isomerised product of step (a) and isolating the base oil from the dewaxed oil. Applicants found that not only the yield to the high viscosity base oil can be improved also the viscosity of the end base oil is higher when processing a feed also containing a diluent. Detailed description of the invention The Fischer-Tropsch wax used as the feed for the present process, is obtained via the well-known Fischer- Tropsch hydrocarbon synthesis process. In general, such Fischer-Tropsch hydrocarbon synthesis involves the preparation of hydrocarbons from a mixture of carbon monoxide and hydrogen at elevated temperature and pressure in the presence of a suitable catalyst. The Fischer-Tropsch catalyst normally is selective for preparing paraffinic molecules, mostly straight-chain paraffins, and the product from a Fischer-Tropsch synthesis reaction therefore usually is a mixture of a large variety of paraffinic molecules. Those hydrocarbons that are gaseous or liquid at room temperature are recovered separately, for instance as fuel gas (C5-) , solvent feedstocks and detergent feedstocks (up to C17) . The more heavy paraffins (C13+) are recovered as one or more wax fractions, commonly referred to as Fischer- Tropsch wax(es) or synthetic wax(es). For the purpose of the present invention only those Fischer-Tropsch waxes are useful as the feed, which meet the aforementioned requirements with respect to its T10wt% recovery boiling point. The Fischer-Tropsch wax may comprise iso-paraffins. The presence of iso-paraffins will however be relatively low. A measure of the amount of iso-paraffins is the oil content of the wax. The wax content as used in the description is measured according to the following procedure. 1 weight part of the to be measured oil fraction is diluted with 4 parts of a (50/50 vol/vol) mixture of methyl ethyl ketone and toluene, which is subsequently cooled to -27 0C in a refrigerator. The mixture is subsequently filtered at -27 0C. The wax is removed from the filter and weighed. If reference is made to oil content a wt% value is meant which is 100% minus the wax content in wt%. The wax content is preferably above 50 wt%, more preferably above 60 wt% and even more preferably between 60 and 100 wt% as measured according to the above method. Within the limits defined hereinbefore, preferred Fischer-Tropsch wax feeds are those having a congealing point in the range of above 80 0C and more preferably between 90 and 150 0C. Those Fischer-Tropsch waxes melting above 90 0C suitably have a kinematic viscosity at a temperature T, which is 10 to 20 0C higher than their melting point, in the range of from 8 to 15 mm^/s, preferably from 9 to 14 mm^/s. The hydrocarbon diluent used in step (a) may in principle be any hydrocarbon mixture having a T90 recovery point of below 400 0C. Preferably the diluent is a paraffin fraction as obtained in the above referred to Fischer-Tropsch synthesis. More preferably it is a fraction boiling for more than 80 wt% between 250 and 400 0C. The paraffin diluent may be a heavy Fischer- Tropsch derived gas oil or a substantially waxy product having a wax content of above 80 wt%, preferably above 90 wt%. Mixtures of such products may also be used. In a preferred embodiment the feed to step (a) is prepared by performing a Fischer-Tropsch synthesis to prepare a Fischer-Tropsch synthesis product. Isolating, by means of distillation, from said Fischer-Tropsch synthesis product the Fischer-Tropsch wax as defined above, an intermediate wax product boiling for more than 80 wt% between 300 and 500 0C and a lower boiling fraction which is used as diluent as defined above. The heavy wax fraction and diluent fraction are combined to form the feed to step (a) . The intermediate wax product is a paraffin wax product having a congealing point of between 45 and 80 or even as high as 90 0C and more preferably between 50 and 85 °C. This wax product is preferably kept separate from the feed to step (a) and is suitably marketed as paraffin wax. The intermediate wax product may be further separated in two or more fractions resulting in wax products having a narrow carbon distribution. The Fischer-Tropsch wax may be subjected to a hydrogenation step prior to the above separation. Hydrogenation, or optionally a mild hydroisomerisation, may suitably be performed on the intermediate wax product after it has been isolated to obtain a marketable wax product. The non-hydrogentated blend can be used directly in step (a) . The hydroconversion catalyst used in step (a) may in principle be any catalyst known in the art to be suitable for isomerising paraffinic molecules. In general, suitable hydroconversion catalysts are those comprising a hydrogenation component supported on a refractory oxide carrier, such as amorphous silica-alumina, alumina, fluorided alumina, molecular sieves (zeolites) or mixtures of two or more of these. Suitable catalysts have been found to be those comprising a Group VIII metal, especially nickel, platinum or palladium and a silica- alumina carrier as will be described in more detail below. One type of preferred catalysts to be applied in the hydroconversion step in accordance with the present invention are hydroconversion catalysts comprising platinum and/or palladium as the hydrogenation component. A very much preferred hydroconversion 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 (calculated as element) 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- 94/10264 and EP-A-O, 582, 347. Other suitable noble metal- based catalysts, such as platinum on a fluorided alumina carrier, are disclosed in e.g. US-A-5, 059, 299 and WO-A-92/20759. A second type of suitable hydroconversion 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. Usually both metals are 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 catalyst. 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. 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. The catalyst has a surface area in the range of 200-500 m2/gm, preferably 0.35 to 0.80 ml/gm, as determined by water adsorption, and a bulk density of about 0.5-1.0 g/ml. The catalyst support is preferably an amorphous silica-alumina where the alumina is present in amounts of less than about 30 wt%, preferably 5-30 wt%,, more preferably 10-20 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. 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 0C, and calcining in air at 200-550 0C. 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 ratio respecting the Group VIII metal. A typical catalyst is shown below:

Ni, wt% 2.5-3.5 Cu, wt% 0.25-0.35 Al2O3-Siθ2 wt% 65-75 Al2O3 (binder) wt% 25-30

Surface Area 290-325 m2/gm Pore Volume (Hg) 0.35-0.45 ml/gm Bulk Density 0.58-0.68 g/ml

Another class of suitable hydroconversion catalysts are those based on zeolitic materials, suitably comprising at least one Group VIII metal component, preferably Pt and/or Pd, as the hydrogenation component. Suitable zeolitic materials, then, include Zeolite beta, Zeolite Y, Ultra Stable Y, ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-35, SSZ-32, ferrierite, zeolite beta, mordenite and silica-aluminophosphates, such as SAPO-Il and SAPO-31 or said list and ZSM-48. Examples of suitable hydroisomerisation catalysts are, for instance, described in WO-A-92/01657. The hydroconversion conditions applied in step (a) are those known to be suitable in hydro-isomerisation operations. Suitable conditions, then, involve operating temperatures in the range of from 275 to 450 0C, preferably 300 to 425 0C, a hydrogen partial pressure in the range of from 10 to 250 bar, suitably 25 to 200 bar, a weight hourly space velocity (WHSV) in the range of from 0.1 to 10 kg/l/h, preferably 0.2 to 5 kg/l/h, and a gas rate in the range of from 100 to 5,000 Nl/kg, preferably 500 to 3,000 Nl/kg. Suitably the conditions in step (a) are so chosen that the wax conversion is preferably between 40 and 90 wt% and more preferably between 60 and 90 wt%. The effluent of step (a) may be directly used as feed to a dewaxing step. Especially if catalytic dewaxing is applied in step (b) it has been found advantageous to perform step (a) and (b) in a series flow configuration, thus without any intermediate separation of lower boiling compounds. Alternatively one may separate from the effluent of step (a) a lighter fraction such to reduce the volume of feed to step (a) . The effective cutpoint of such a separation or said otherwise of the heavy remaining fraction is suitably in the range of from 400 to 550 0C. The effective cutpoint of the heavy fraction is the temperature above which at least at least 85% by weight and preferably at least 90% by weight, of the hydrocarbons present in this heavy fraction has its boiling point. This separation or fractionation can be achieved by techniques known in the art, such as atmospheric and vacuum distillation or vacuum flashing. In step (b) the base oil precursor fraction obtained in step (a) is subjected to a pour point reducing treatment. With a pour point reducing treatment is understood every process wherein the pour point of the base oil is reduced by more than 10 0C, preferably more than 20 0C, more preferably more than 25 0C. The pour point reducing treatment can be performed by means of a so-called solvent dewaxing process or by means of a catalytic dewaxing process. Solvent dewaxing is well known to those skilled in the art and involves admixture of one or more solvents and/or wax precipitating agents with the base oil precursor fraction and cooling the mixture to a temperature in the range of from -10 0C to -40 0C, preferably in the range of from -20 0C to -35 0C, to separate the wax from the oil. The oil containing the wax is usually filtered through a filter cloth which can be made of textile fibres, such as cotton; porous metal cloth; or cloth made of synthetic materials. Examples of solvents which may be employed in the solvent dewaxing process are Cβ-Cg ketones (e.g. methyl ethyl ketone, methyl isobutyl ketone and mixtures thereof) , Cg-C^o aromatic hydrocarbons (e.g. toluene), mixtures of ketones and aromatics (e.g. methyl ethyl ketone and toluene), autorefrigerative solvents such as liquefied, normally gaseous C2-C4 hydrocarbons such as propane, propylene, butane, butylene and mixtures thereof. Dichloromethane and mixtures of methyl ethyl ketone and toluene or methyl ethyl ketone and methyl isobutyl ketone are generally preferred. Examples of these and other suitable solvent dewaxing processes are described in Lubricant Base Oil and Wax Processing, Avilino Sequeira, Jr, Marcel Dekker Inc., New York, 1994, Chapter 7. The slack wax obtained in the solvent dewaxing treatment of step (b) is suitably recycled, i.e. all or part of this slack wax is routed back to the hydroconversion step (a) , most conveniently by blending it with the fresh Fisher-Tropsch wax feed, provided the feed characterisation is still within the definition according to the present invention. In this way the final yield of lubricating base oil can be maximised. Preferably step (b) is performed by means of a catalytic dewaxing process. With such a process it has been found that base oils having a pour point of even below -40 0C can be prepared when starting from a base oil precursor fraction as obtained in step (a) of the present process. The catalytic dewaxing process can be performed by any process wherein in the presence of a catalyst and hydrogen the pour point of the base oil precursor fraction is reduced as specified above. Suitable dewaxing catalysts are 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 the 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-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 Pt/ZSM-35, 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-4343692, US-A-5053373, US-A-5252527 and US-A-4574043. The dewaxing catalyst suitably also comprises a binder. The binder can be a synthetic or naturally occurring (inorganic) substance, for example clay, silica and/or metal oxides. Natural occurring clays are for example of the montmorillonite and kaolin families. The binder is preferably a porous binder material, for example a refractory oxide of which examples are: alumina, silica-alumina, silica-magnesia, silica- zirconia, silica-thoria, silica-beryllia, silica-titania as well as ternary compositions for example silica- alumina-thoria, silica-alumina-zirconia, silica-alumina- magnesia and silica-magnesia-zirconia. More preferably a low acidity refractory oxide binder material, which is essentially free of alumina, is used. Examples of these binder materials are silica, zirconia, titanium dioxide, germanium dioxide and mixtures of two or more of these of which examples are listed above. The most preferred binder is silica. A preferred class of dewaxing catalysts comprise intermediate zeolite crystallites as described above and a low acidity refractory oxide binder material which is essentially free of alumina as described above, wherein the surface of the aluminosilicate zeolite crystallites has been modified by subjecting the aluminosilicate zeolite crystallites to a surface dealumination treatment. A preferred dealumination treatment is by contacting an extrudate of the binder and the zeolite with an aqueous solution of a fluorosilicate salt as described in for example US-A-5157191 or WO-A-0029511. Examples of suitable dewaxing catalysts as described above are silica bound and dealuminated Pt/ZSM-5, silica bound and dealuminated Pt/ZSM-23, silica bound and dealuminated Pt/ZSM-12, silica bound and dealuminated Pt/ZSM-22, as for example described in WO-A-0029511 and EP-B-832171. Catalytic dewaxing conditions are known in the art and typically involve operating temperatures in the range of from 200 to 500 0C, suitably from 250 to 400 0C, 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/l/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, suitably between 315 and 375 0C at between 40-70 bars, in the catalytic dewaxing step it is possible to prepare base oils having different pour point specifications varying from suitably -10 to -60 0C. Optionally steps (a) and (b) may be performed in one step using the same catalyst comprising a molecular sieve. Optionally steps (a) and (b) can be performed using a molecular sieve comprising catalyst in both steps wherein the pore opening of the molecular sieve in step (a) is larger than the molecular sieve used in step (b) suitably making use of the catalysts as exemplified at steps (a) and (b) above. The invention is now further illustrated by the following examples without restricting the scope of the invention to these specific embodiments. In the experiments use was made of wax feedstocks A and D. These wax feedstocks were prepared by hydrogenation of a Fischer-Tropsch synthesis product followed by distillate separation using wiped film evaporators. Products B and C are the intermediate wax products as discussed also above. These wax products B and C having a narrow carbon distribution may find application in for example hot melt adhesives, printing inks, cable filling, match sticks, corrugated board, fibre board and PVC lubricants. The unique white colour of these waxes make them ideal for application requiring colour additive, e.g. crayons, candles, graphic arts and other decorative items. The opaque appearance produces true colour brilliance with minimum colouring agents.

Table 1 n.a.= not analysed

Comparative experiment A A Fischer-Tropsch wax feed B having the properties as listed in Table I was contacted with a fluorided NiW/alumina catalyst (5.0 %wt Ni, 23.1 %wt W, 4.6 %wt F, all based on total weight of carrier) at a temperature ranging from 370 to 400 0C, a hydrogen partial pressure of 140 bar, a WHSV of 1 kg/l/h and a gas rate of 1,500 Nl/kg. The effluent was fractionated and the 390 °C+ fraction (obtained at a yield of 87.8% by weight based on total effluent) was subsequently solvent dewaxed using MEK/toluene at -20 0C. It was found that the final base oil yield had a maximum at 388 0C reactor temperature. In table 2 the results are presented. Comparative experiment B Experiment A was repeated for feed C at various reactor temperatures ranging from 383 to 399 0C. It was seen that the final base oil yield had a maximum at 389 0C reactor temperature. In table 2 the results are presented. Comparative experiment C Experiment A was repeated for feed D at a reactor temperature of 409 and 420 °C. At 409 a maximum base oil yield was observed. In table 2 the results are presented, Example 1 Experiment A was repeated except that the feed consisted of 20 wt% of feed A and 80 wt% of feed D. The reactor temperature ranged from 403 to 420 0C. The results are presented in Table 2. Table 2

Example 2 The Fischer-Tropsch feed as used in Example 1 was contacted with a PtPd/ASA (0.3 %wt Pt, 1 %wt Pd, ASA: silica/alumina molar ratio is 55/45) catalyst at a different temperatures, whilst the other conditions were the same as applied in Example 1. The effluent was fractionated and the 390 °C+ fraction and the residue was subsequently solvent dewaxed using MEK/toluene at -20 0C. The results are presented in Figure 1. In this Figure also some results of Examples according to Example 1 are presented for comparison reasons. The oil obtained at the highest yield had a VI of 150, a pour point of -24 0C, a kinematic viscosity at 100 0C (VkIOO) of 9.161 mm2/s and a Noack volatility of 5.8% by weight. Example 3 As Example 2 but at a hydrogen partial pressure of 90 bar. See Figure 1 for results. The oil obtained at the highest yield had a VI of 146, a pour point of -27 0C, a kinematic viscosity at 100 0C (VkIOO) of 8.13 mm2/s and a Noack volatility of 7.2% by weight. ' Example 4 As Example 2 but at a hydrogen partial pressure of 40 bar. See Figure 1 for results. The oil obtained at the highest yield had a VI of 151, a pour point of -24 0C, a kinematic viscosity at 100 0C (VkIOO) of 8.317 mm2/s and a Noack volatility of 6.7% by weight.