An algorithm by which a time normalized oil condition trend (OCT) is developed. During engine off periods, while the engine is cooling, the vehicle's main computer program implements the trend algorithm according to the present invention by which engine oil conductivity (oil sensor output voltage) and temperature data are obtained in a specified temperature range, and are then input into a cool down equation to determine its coefficients using nonlinear regression. This cool down equation models oil conductivity as a function of temperature and is used at a specified time during an engine-on period to calculate an OCT point. Collectively, these OCT points determine the oil condition trend that is analyzed by a procedure called from the vehicle's main computer program to determine when an oil change is necessary.