The presence of fuel in the crankcase reduces oil viscosity and weakens detergency. Excessive amounts of the contaminant can so diminish the oil's lubrication properties that engine failure will result. Although some fuel dilution is expected, amounts in excess of three  percent in two cycle engines and four percent in four cycle engines must be considered abnormal. These levels indicate mechanical problems generating from defects in injectors, fuel pumps, gaskets or seals. Fuel dilution is detected in the laboratory by three methods: viscosity, flash point and gas chromatography. Viscosity of the contamination oil will usually drop from the original or new oil viscosity. However, other factors such as fuel soot levels and oil oxidation can cancel this effect by increasing viscosity. Also, additive shear in multi-weight oils can add to viscosity drop.

Flash point of the oil is a better way to quantify percentages of fuel contamination. This test measures the temperature at which the oil will ignite when an open flame is applied to it. Since diesel fuel typically flashes at approximately 130 F° and engine oil flashes at approximately 420 F° , the amount of fuel can be estimated by the decrease in used oil flash as compared to new. One caution advised when using this method to determine percent fuel dilution is that excessive antifreeze contamination will also reduce flash point. Therefore, if antifreeze contamination is indicated, flash point cannot be relied upon to accurately measure fuel percentages.

The most accurate method for a quantitative measurement of fuel dilution is gas chromatography. This is a method of separating the contents of the sample by boiling point. The accuracy of this method is 0.1% and is normally repeatable to the nearest 0.5%.