(where R is an alkenyl group, R1 is a monovalent hydrocarbon group having no fatty unsaturated bonds, R2 is a monovalent fatty hydrocarbon group (R2 containing at least 50 mol % of methyl group and, if included, not higher than 10 mol % of alkenyl group), and n is such a numeral that the viscosity ranges from 100 to 100,000 cSt at 25??? C.); and