Oils containing copolymers

A composition comprises an oil and an oil-soluble copolymer of an N-vinyl butyrolactam and at least one copolymerizable organic compound. The N-vinyl butyolactam is represented by the general formula wherein R is hydrogen or a C1-C12 alkyl group, but wherein at least one of the R groups must be hydrogen, and R1 is hydrogen or a C1-C12 alkyl group. The preferred compound is N-vinyl-a -pyrrolidone, i.e. where both R and R1 in the general formula are hydrogen. Preferred compounds which are copolymerized with the N-vinyl butyrolactam to produce the additive compound are polymerizable unsaturated esters, derived from unsaturated acids or alcohols containing an unsaturated carbon to carbon linkage capable of forming a polymeric linkage when reacted with the lactam; the preferred esters are esters of C3-C8 organic acids such as vinyl acetate. As copolymerizable compounds there may also be used ethers or hydrocarbons, particularly olefins or polymerized olefins such as ethylene or alkylated styrene. The composition may be prepared as a concentrate and may be added to mineral oils, particularly lubricating and fuel oils, to vegetable oils or to synthetic lubricating oils, particularly those comprising complex esters or diesters such as di-2-ethyl hexyl sebacate. The composition may also contain conventional fuel or lubricating oil additives such as oil-soluble sulphonates, metal salts of alkyl phenol sulphides and extreme-pressure, anti-wear and anti-oxidant additives.