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Accumulation of fuel, water, acids, insolubles, and metals in engine oil is documented and compared for variable-fueled (fuel containing up to 85 percent methanol) and gasoline-fueled vehicles in short-trip service. The oil temperature at which various contaminants are removed is noted. As a consequence of emulsion formation, the viscosity of the oil in the M85-fueled vehicles increased. Due to the presence of gasoline, the viscosity of the oil in the gasoline-fueled vehicles decreased. Equations were developed to explain both the viscosity reduction due to gasoline and the viscosity increase due to emulsion-forming contaminants (water and methanol).

The results of several investigations indicate the extent to which driving cycle, oil formulation, and fuel type (either regular unleaded gasoline or M85) influence the nature and severity of engine-oil degradation and engine damage. Driving cycle greatly influenced mass loss of piston rings and main and connecting rod bearings. For example, short-trip, cold start service with M85 caused 80 times more wear of top piston rings per km of service than was observed in long-trip service with the same oil. The magnitude of engine oil degradation was also documented. Under freeway driving conditions, in which the engine oil warmed completely, service with M85 fuel caused approximately the same amount of oil degradation as was found with gasoline. In city service, several engine oil parameters (base number, accumulation of insoluble contaminants, viscosity) degraded twice as fast with gasoline as with M85.