The calculation of reliable indexes of oil contamination caused by engine wear is necessary in order to use oil analysis as a failure diagnostic tool. In this paper, a method for the correction of the measurements obtained from oil spectrometric analysis is proposed. A realistic model must consider the combined effects of the oil filter efficiency, the oil consumption and the amount of fresh oil added. The concentration of contaminants is normalized taking into account all the above effects with the assumption of a constant contamination rate.At the beginning of the study the repeatability of the measuring equipment and the effect of the method of obtaining the samples were calculated. Three factors were considered: the location which the sample was taken from (the oil pan drain plug or the oil level dipstick orifice), the engine temperature (cold and warm) and the time delay from sample taking to its analysis.To validate the model, several studies were carried out: An experiment was made to determine the effect of commercial filters and another experiment was scheduled to simulate the oil consumption and the addition of fresh oil.