A lubricating oil is obtained from an aromatic extract obtained from kerosine and having an initial boiling-point between 140 DEG and 160 DEG C. and a final boiling-point between 260 DEG and 280 DEG C. and a C8-C18 mixture of olefins obtained by cracking heavy paraffin hydrocarbons separating the aromatic extract by distillation into two fractions, the final boiling-point of one fraction and the initial boiling-point of the other fraction being between 200 DEG and 245 DEG C., separating the mixture of olefins by distillation into a C8 - Cn and a Cn+1 - C18 fraction (wherein n is any integer from 11 to 17 inclusive), alkylating the lower boiling fraction of the aromatic extract, in the presence of an alkylation catalyst, with the higher boiling fraction of the olefins in a ratio of at least two mols. of aromatic compounds per mol. of olefins, alkylating the higher boiling fraction of the aromatic extract in the presence of an alkylation catalyst with the lower boiling fraction of the olefins in a ratio of at least two mols. of olefins per mol. of aromatic compounds, and mixing the two alkylation products thus obtained in such proportions as to yield a product with a viscosity (E50) within the range 6 to 18. The product of the first alkylation is a thin lubricating oil with a viscosity index of 95 to 110 and a sufficiently high flash-point (above 200 DEG C.) whilst the second alkylation yields a thick lubricating oil with a viscosity index of 95 to 105 and a sufficiently high flash point. The aromatic extract used in the process can be obtained from kerosine by extraction with a selective solvent such as furfural, nitrobenzene, or antimony trichloride, and especially liquid sulphur dioxide. The aromatic compounds may also be separated from kerosine by other methods, e.g. a kerosine diluted with a light hydrocarbon oil such as pentane may be percolated over silica gel and the adsorbed aromatic compounds then dissolved off in a volatile solvent such as pentane. The olefins may be obtained by cracking paraffin wax in the vapour phase and distilling the C8-C18 fraction out of the cracked product. The bulk of the most reactive components such as the di-olefins are preferably removed from the cracked product before distilling of the C8-C18 fraction by contacting the cracked product with a small amount of aluminium chloride, preferably at 60 DEG to 90 DEG C. so as to polymerize the reactive components. Any Friedel-Crafts catalyst may be employed for the alkylation, e.g. HF, BF3, FeCl3, ZnCl2 and H2SO4 and especially aluminium chloride. The first alkylation is usually effected at 20 DEG to 100 DEG C. whilst the second alkylation is generally effected at -10 DEG to +100 DEG C. The upper layer resulting from each alkylation can be treated with an alkaline substance such as lime to remove acid components and the alkylation products may also be further purified by treating with bleaching earth or the two treatments may be combined by heating the alkylation product with a mixture of bleaching earth and lime at 150 DEG to 240 DEG C., preferably in an inert atmosphere, e.g. of nitrogen. In an example an aromatic extract obtained from kerosine by extraction with sulphur dioxide is separated into a fraction of boiling range 160 DEG to 210 DEG C. and a fraction of boiling range 210 DEG to 260 DEG C. and a mixture of C8-C18 olefins, obtained by cracking a residual oil-containing paraffin wax in the vapour phase and treating with aluminium chloride at 80 DEG C. to remove the most reactive components, is separated into a C8-C11 fraction and a C12-C18 fraction. The lighter fraction of the kerosine extract is alkylated with the C12-C18 olefins and the heavier fraction alkylated with the C8-C11 olefins, the alkylation being effected in each case at 45 DEG C. in the presence of aluminium chloride, the first alkylate is distilled to give an oil of viscosity (E50) of about 4 and the second alkylate distilled to yield an oil of viscosity (E50) of 25 the two oils being then mixed to produce the final lubricating oil. The procedure is repeated using C8-C13 and C14-C18 olefin fractions, C8-C15 and C16-C18 fractions, and C8-C17 and C18 fractions and the alkylates mixed as before to produce the lubricating oil. The viscosity (E50) and flashpoint of the resulting lubricating oil mixture is given in each case and these values are compared with those of the oil obtained by alkylating the aromatic extract with the C8-C18 olefine mixture without prior separation of either reactant into fractions. The performances of the following oils in engine tests are also compared: (a) an oil obtained by alkylating an aromatic extract fraction of boiling range 160 DEG to 210 DEG C. with a C14-C18 olefin mixture and distilling the product to a viscosity E(50) of 4; (b) the product obtained by alkylating an aromatic extract of boiling range 210 DEG to 260 DEG C. with a C8-C13 mixture of olefins and distilling to yield a product of viscosity E(50) of 25; (c) a mixture of (a) and (b) having a viscosity E(50) of 18. Before testing oil (a) the lighter components are distilled off to yield a residue of viscosity E (50) of 14. Specifications 665,008 and 665,058 are referred to.ALSO:A lubricating oil is obtained from an aromatic extract obtained from kerosine and having an initial boiling point between 140 DEG and 160 DEG and a final boiling point between 260 DEG and 280 DEG C and a C8-C18 mixture of olefins obtained by cracking heavy paraffin hydrocarbons, separating the aromatic extract by distillation into two fractions the final boiling point of one fraction and the initial boiling point of the other fraction being between 200 and 245 DEG C, separating the mixture of olefins by distillation into a C8-Cn and a Cn+1-C18 fraction (wherein n is any integer from 11 to 17 inclusive), alkylating the lower boiling fraction of the aromatic extract, in the presence of an alkylation catalyst, with the higher boiling fraction of the olefins in a ratio of at least two mols of aromatic compounds per mol of olefins, alkylating the higher boiling fraction of the aromatic extract in the presence of an alkylation catalyst with the lower boiling fraction of the olefins in a ratio of at least two mols of olefins per mol of aromatic compounds, and mixing the two alkylation products thus obtained in such proportions as to yield a product with a viscosity (E50) within the range 6 to 18. The product of the first alkylation is a thin lubricating oil with a viscosity index of 95 to 110 and a sufficiently high flash point (above 200 DEG C) whilst the second alkylation yields a thick lubricating oil with a viscosity index of 95 to 105 and a sufficiently high flash point. The aromatic extract used in the process can be obtained from kerosine by extraction with a selective solvent such as furfural, nitrobenzene, or antimony trichloride and especially liquid sulphur dioxide. The aromatic compounds may also be separated from kerosine by other methods, e.g. a kerosine diluted with a light hydrocarbon oil such as pentane may be percolated over silica gel and the adsorbed aromatic compounds then dissolved off in a volatile solvent such as pentane. The olefins may be obtained by cracking paraffin wax in the vapour phase and distilling the C8-C18 fraction out of the cracked product. The bulk of the most reactive components such as the di-olefins are preferably removed from the cracked product before distilling off the C8-C18 fraction by contacting the cracked product with a small amount of aluminium chloride preferably at 60 to 90 DEG C so as to polymerize the reactive components. Any Friedel-Crafts catalyst may be employed for the alkylation, e.g. HF, BF3, FeCl3, ZnCl2 and H2SO4, and especially aluminium chloride. The first alkylation is usually effected at 20 to 100 DEG C whilst the second alkylation is generally effected at -10 DEG to +100 DEG C. The upper layer resulting from each alkylation can be treated with an alkaline substance such as lime to remove acid components and the alkylation products may also be further purified by treating with bleaching earth or the two treatments may be combined by heating the alkylation product with a mixture of bleaching earth and lime at 150 DEG to 240 DEG C preferably in an inert atmosphere, e.g. of nitrogen. In an example, an aromatic extract obtained from kerosine by extraction with sulphur dioxide is separated into a fraction of boiling range 160 DEG to 210 DEG C and a fraction of boiling range 210 DEG to 260 DEG C, and a mixture of C8-C18 olefins, obtained by cracking a residual oilcontaining paraffin wax in the vapour phase and treating with aluminium chloride at 80 DEG C to remove the most reactive components, is separated into a C8-C11 fraction and a C12-C18 fraction. The lighter fraction of the kerosine extract is alkylated with the C12-C18 olefins and the heavier fraction alkylated with the C8-C11 olefins, the alkylation being effected in each case at 45 DEG C in the presence of aluminium chloride, the p first alkylate is distilled to give an oil of viscosity (E50) of about 4 and the second alkylate distilled to yield an oil of viscosity (E50) of 25, the two oils being then mixed to produce the final lubricating oil. The procedure is repeated using C8-C13 and C14-C18 olefin fractions, C8-C15 and C16-C18 fractions, and C8-C17 and C18 fractions and the alkylates mixed as before to produce the lubricating oil. The viscosity (E50) and flash point of the resulting lubricating oil mixture is given in each case and these values are compared with those of the oil obtained by alkylating the aromatic extract with the C8-C18 olefine mixture without prior separation of either reactant into fractions. The performances of the following oils in engine tests are also compared: (a) an oil obtained by alkylating an aromatic extract fraction of boiling