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Conventionally, the engine oils for passenger cars have been used as four stroke cycle gasoline engine oils for motorcycle from old-time. However, the engine oils for passenger cars have been required to have fuel economy performance, and these engine oils are not fit in the requirement of motorcycles because of low friction and low viscosity. For this reason the four stroke cycle gasoline engine oils for motorcycles have been classified into two categories of ‘MA’ and ‘MB’ based on differences in friction characteristics according to standard of Japan Automobile Standards Organization (hereinafter referred to as ‘JASO’) -JASO T 903:1998. In currently, some engine oils for passenger cars can fit in JASO standard, but the range of friction coefficient in MA category is becoming wider.

With environmental issues at the forefront of the automotive industries, there is a clear need to improve the fuel economy and longer drain performance of four-cycle motorcycle engine oils. Evaluation of oil consumption in actual engines was used to clarify an optimal volatility for engine oils, and the feasibility of using zinc dialkyl phosphate, a sulfur-free additive, as an alternative to ZDTP for improved of fuel economy performance was studied. Engine oils with optimized additives demonstrated excellent friction characteristics and wet clutch performance.