PURPOSE: To provide the title oil having adequate viscosity, to be suitably used in an atmosphere of hydrogen-contg. fluoroethane cooling medium, containing, as the base oil, an ester prepared by reacting an aliphatic polyhydric alcohol with a saturated aliphatic monocarboxylic acid (derivative).

CONSTITUTION: The objective oil containing, as the base oil, an ester 140-600 in molecular weight and 1-10 (pref. 2-6) cst in kinematic viscosity at 100°C which is prepared by reaction between (A) an aliphatic polyhydric alcohol having 1-6 primary hydroxyl group(s) (pref. a hindered alcohol) and (B) a 2-9C saturated aliphatic monocarboxylic acid or its derivative.