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Polymeric mineral oil additives are exposed to mechanical and thermo-oxidative degradation processes in the engine. For long lasting efficiency, they must show the highest possible stability to such attacks. The degradation processes are evident from and distinguishable by the molecular weight distribution of the polymer which can be measured with great accuracy by gel permeation chromatography (GPC), even for used oils. Results of such measurements are given for various types of VI improvers (polyalkyl methacrylates, ethylene-propylene copolymers and hydrogenated styrene-diene block copolymers) after exposure to Diesel and Otto engines. The influence of mechanical and thermo-oxidative degradation is discussed by means of model calculations. Comparative viscosity data elucidate the picture obtained from the distribution measurements.