A method of improving the seal compatibility and/or copper corrosion performance of lubricating oil compositions for the lubrication of the crankcases of an internal combustion engine, which method includes the step of adding to the lubricating oil compositions a minor amount of a non-hydrogenated (unsaturated) olefin polymer. Also described are lubricating oil compositions for engines and transmissions, which compositions contain sulphur and/or a salicylate soap and a minor amount of a non-hydrogenated (unsaturated) polymer, which compositions are compatible with nitrile rubber engine and transmission seals and copper-containing engine and transmission components.