PURPOSE: To provide the subject base oil containing neopentyl polyol and a synthetic ester produced from neo acid, etc., at a specific ratio, having excellent stability to heat and oxidation and exhibiting high hydrolysis stability.

CONSTITUTION: The objective oil contains (A) a neopentyl polyol and (B) ≥10wt.% of a synthetic ester produced by using (i) ≥10mol% of a neo acid of formula (R is 1-8C alkyl; R1 and R2 are 1-2C alkyl) and (ii) ≤90mol% of a monocarboxylic acid other than neo acid as constituent fatty acid. The component A is e.g. neopentyl glycol, trimethylolethane, trimethylolpropane and (di) pentaerythritol.