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1 Fuel savings by engine oil have been requested for two-wheeled vehicles from the viewpoint of environmental issues. In four-wheeled vehicles, reduction of oil viscosity and addition of friction modifiers have been effective in improving fuel efficiency. However, direct application of engine oil for four-wheeled vehicles to two-wheeled vehicles is difficult. In a four-cycle two-wheeled vehicle, the transmission, gears, and a wet clutch system are imbedded within the engine1). Engine oil must display a remarkable performance as it is required to function as transmission oil and to improve anti-metal fatigue life and clutch performance2), 3). If fuel efficiency is improved by reducing the viscosity of engine oil used in two-wheeled vehicles, the fatigue life tends to worsen. Therefore, reduction in oil viscosity is difficult to achieve.

As well as a four-wheeled vehicle, in the field of motorcycle, development of the CO₂ reduction technology and practical use are required for global environment protection. Therefore the energy conserving by engine oil is required. There are several technologies which have been applied to the passenger car engine oils using lower viscosity base oils and some friction modifier additives. However, these technologies might bring difficulties or problems to apply to the motorcycle engine oils. Since motorcycles normally have transmission gears and a wet clutch system inside the crankcase, the gear lubricity and clutch friction performance are additionally required for motorcycle engine oils. Motorcycle engine oils are also used under severer conditions than passenger car engine oils. Therefore, lower volatility performance at higher temperature conditions, higher shear stability, anti-wear performance and gear pitting toughness are needed.

As well as a four-wheeled vehicle, in the field of motorcycle, development of the CO2 reduction technology and practical use are required for global environment. Zinc dialkyldithiophosphate (Zn-DTP) type additives are widely used for engine oil formulations. However, phosphorus compounds are environmental load materials. The reduction of the quantity of phosphorus compounds in engine oils is required to reduce poisoning of three-way catalysts used to purify exhaust gases from internal combustion engines. Ito1) reported that they developed a sulfur-based additive as a substitute for Zn-DTP. Their non-phosphorus engine oil formulation for four-wheeled vehicles with a sulfur-based additive was examined to evaluate its anti-wear performance using the following test methods: JASO M328 for gasoline engines (KA24E) and JASO M354 for Diesel engine (4D34T4).