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<div class="section abstract"><span class="label">1</span><div class="htmlview paragraph">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 engine^{<span class="xref">1</span>)}. 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 performance^{<span class="xref">2</span>), <span class="xref">3</span>)}.</div><div class="htmlview paragraph">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. In this study, the anti-metal fatigue life of an engine oil was evaluated using a gear or a rolling bearing in a crankshaft where fatigue failures may occur due to the reduction in oil viscosity. Measures to prolong fatigue life were examined by testing different engine oil formulations, including single-grade oils and multi-grade oils to which polymers were added as viscosity index improvers (VIIs). The viscosity characteristics and oil film formability of test oils were measured, and their effects on the fatigue life were analyzed. The use of olefin co-oligomer as a VII improved the oil film formability of engine oil and prolonged the fatigue life of the needle bearing.</div></div>