The olefin oligomer to be used in this invention has the formula (I): ##STR3## 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 (e.g., straight chain and branched chain alkyl groups such as a hexyl group, a heptyl group, an octyl group, a nonyl group and a decyl group) and n represents the degree of polymerization