Abstract: A method for advising a motor vehicle operator of the need to change the lubricating oil in a direct or an indirect injection diesel engine. The rate of degradation of the engine oil is determined from monitoring engine revolutions, engine oil temperature and engine oil contamination content. At the start of service after an oil change has occurred, a value corresponding to the maximum allowed number of engine revolutions for the useful life of the oil is stored in the memory of the vehicle's computer. Periodically during each period of vehicle operation, an effective engine revolutions value is determined in relation to the product of measured engine revolutions, an engine oil temperature dependent penalty factor and an oil contaminant content dependent penalty factor. The penalty factors increase the effective engine revolutions value to compensate for engine operating conditions that tend to cause increased degradation of the engine oil.