Abstract: The molecular weight and the thickening efficiency of primarily oil soluble hydrocarbon polymers, such as hydrogenated random copolymers and block copolymers of styrene-isoprene or styrene-butadiene, polyisobutylene, poly-n-butene, low unsaturation butyl rubbers, low unsaturation EPDM rubbers, preferably ethylene-propylene copolymers, of the type which are to be used as viscosity index improver additives or as precursors for dispersants and multi-functional viscosity improver additives for lubricating oils, are reduced by heating and masticating the polymers in the presence of oxygen or air and in the presence of a transition metal catalyst, preferably in the further presence of a peroxide accelerator.