A lube base oil composition which comprises 80 to 98 wt.% (a) naphthenic synthetic lube base oil having a flash point of 140°C or higher and 2 to 20 wt.% (b) fatty acid ester of an alcohol having a gem-dimethyl structure, the ester having a flash point of 150°C or higher; or a lube base oil composition which comprises 85 to 99 wt.% (a) naphthenic synthetic lube base oil having a flash point of 140°C or higher and 1 to 15 wt.% (c) mineral oil having a dynamic viscosity as measured at 40°C of 0.9 to 9.6 mm2/s, a boiling point as measured at ordinary pressure of from 150 to 400°C, and a %CA as determined through ring analysis (n-d-M method) of 10% or lower. A traction drive fluid is provided which has a flash point of 150°C or higher, a high coefficient of traction at high temperatures, and excellent flow properties at low temperatures.