To improve anti-oxidation stability and heat stability of a base oil by making the base oil contain a specific polyether-based lubricating base oil and a phenol-based or amine-based antioxidant.
A base oil is obtained by mixing 0.01-5 wt.%, preferably 0.05-3 wt.% of a phenol-based or amine-based antioxidant with a specific polyether- based lubricating base oil represented by the formula having 15-200 cSt, preferably 20-150 cSt, an unsaturation degree of not more than 0.05 meq/g, preferably not more than 0.04 meq/g, a peroxide value of not more than 10.0 meq/kg, preferably not more than 5 meq/kg and a carbonyl value of not more than 100 weight ppm, preferably not more than 50 weight ppm, where, in the formula, X is a mono-ol or a residual group obtained by removing hydroxy groups from a polyol, (AO)n is a polyoxyalkylene obtained by polymerization of an alkyleneoxide having not less than 3 carbon atoms or copolymerization of ethylene oxide and an alkyleneoxide having not less than 3 carbon atoms, R is H or a 1-4C alkyl and (n) is not less than 2.
NAMIKI NAOTO
NANIWA KIMIYOSHI
MINOWA YOSHIO
SANEYOSHI AYAKO
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