The present invention provides: an oil agent additive that is easily soluble in an oil agent and is outstandingly effective in reducing the friction coefficient; and an oil agent composition that contains the oil agent additive. This oil agent additive contains at least one compound represented by a Chemical Formula (1). (In the formula: R1 and R2 are each an aliphatic hydrocarbon group with a carbon number of 1-33; the total carbon number of R1 and R2 is 2-34; X is a single bond or an aliphatic hydrocarbon group with a carbon number of 1-5; and A is —O—CH2—CH(OH)—CH2OH or —O—CH(—CH2—OH)2.)