The advent and high market share of the flex fuel were viable also due to the use of software to identify the type of fuel via the oxygen sensor signal manipulation. However, these routines are subjected to a non observable system, especially in conditions where the fuel enters the combustion chamber by ways other than the injectors, as the blow-by system, which leads the gases from oil pan. These gases can present high alcohol concentration, which evaporates from the contaminated oil, mainly after starts at low temperatures, causing the shifting of the oxygen sensor signal and consequently the misinterpretation of the fuel type flowing into the engine. This paper has the objective to propose an estimation method for the mass of fuel in the oil, using separate simplified physical models for the oil contamination and for the alcohol evaporation processes. The results show that an on-line estimation can be performed with a relatively simple parametric model with a defined calibration methodology.