To provide a base oil for a four-cycle engine oil, having characteristics of stably retaining an oil film under a severe condition for a long time, even though no additive is mixed thereinto, and capable of enhancing a rocker arm pad scuffing preventive performance, a camshaft abrasion preventive performance, and a camshaft pitching preventive performance.
This base oil for the four-cycle engine oil comprises (A) an ester-based lubricant base oil in an amount of 10-90 wt% based on the total weight of the base oil and (B) a mineral oil-based lubricant base oil and/or a poly-α-olefin-based lubricant base oil in an amount of 90-10 wt% based thereon. Further, the ester-based base oil (A) comprises (A-1) a copolymer of a dicarboxylic acid ester and a 3-20C α-olefin having a kinematic viscosity of 20-50 mm2/s at 100°C in an amount of 5-30 wt% and (A-2) an ester of an polyhydric alcohol with a fatty acid and having a kinematic viscosity of 2-50 mm2/s at 100°C in an amount of 5-60 wt%.
Akihiko Yamashita
Kishimoto Tatsuto
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