It is proposed that alkylated polystyrene is suitable for use as a viscosity index improver for lubricating oils. It can be prepared by alkylating polystyrene with tertiary alkyl chlorides or bromides from 4 to 8 carbon atoms in the presence of aluminium chloride or bromide as catalyst while having the polystyrene dissolved in an inert solvent. It was observed that as the degree of alkylation increases the polymer becomes more soluble in oil, consequently the viscosity index improving performance of the polymer increases. It was also seen that as the size of the alkyl group attached to the polystyrene ring increases, the viscosity index, VI, improving performance of the polymer decreases. Molecular weight of a polymer has a great influence upon the VI improving effect of the polymer. As it is increased the thickening performance also increases. The experimental data showed that the thickening effect of a polymeric viscosity index improver decreases with increasing viscosity of the base oil.