A composition comprising (a) poly-4-methylpentene-1 and (b) an olefin oligomer of the formula (I): ##STR7## wherein R1, R2, R3 and R4, which may be the same or different, each is a hydrogen atom or an alkyl group having up to 18 carbon atoms, and n represents the degree of polymerization and is such that the oligomer has an average molecular weight of about 200 to about 5,000, said oligomer being