Abstract: A method and an engine are disclosed for determining an amount of fuel in the engine oil. Such information can be used to recommend an oil change and to estimate a fraction of biodiesel in the fuel supplied to the engine. According to one embodiment, actual oil volume in the oil pan is based on a signal from a level sensor in the oil and a theoretical oil volume is determined based on initial oil volume and fuel entering the oil in the oil pan, fuel leaving the oil pan due to evaporation, and oil being consumed in the combustion chamber. Based on the difference between the actual and theoretical oil volumes, a proportion of biodiesel can be determined and transmitted to an engine control system.