Abstract: Polyalkyl acrylates and methacrylates, suitable for improvement of the viscosity index, are used as oil additives imparting improved dispersant and detergent effects in diesel and gasoline engines, said polyalkyl acrylates or methacrylates being copolymers comprising from 80 to 99.5 parts by weight of alkyl acrylates of methacrylates of the formula ##STR1## wherein R is hydrogen or methyl and R.sub.1 is alkyl having from 6 to 24, and preferably from 8 to 20, carbon atoms, together with from 0.5 to 20 parts by weight of at least one functionalized alkyl methacrylate of the formula ##STR2## wherein R.sup.1 is hydrogen or methyl and R.sub.2 is alkyl having from 2 to 6 carbon atoms and substituted with at least one OH group, or is a polyalkoxylated group of the formula ##STR3## wherein R.sub.3 and R.sub.4 are hydrogen or methyl, R.sub.