A four-cycle engine oil formulation with outstanding fuel economy and clutch-slippage prevention has been developed for motorcycles. Fuel economy performance was evaluated by testing low-friction formulations with a motor-driven torque measurement tester and an SRV reciprocating friction tester. An optimized formulation containing both a molybdenum compound and a phosphorus extreme-pressure agent was found to give the best low-friction performance. The further addition of a boron-type dispersant maintained the low-friction performance while preventing clutch slippage. When test-produced oils were used for 1800 km in a commuter motorcycle, the fuel economy improved by 6 to 7% compared with an existing commercial oil.