High concentrations of diesel fuel can accumulate in the engine oil, especially in vehicles equipped with diesel particle filters. Fuel dilution can decrease the viscosity of engine oil, reducing its film thickness. Higher concentrations of fuel are believed to accumulate in oil with biodiesel than with diesel fuel because biodiesel has a higher boiling temperature range, allowing it to persist in the sump. Numerous countries are taking actions to promote the use of biodiesel. The growing interest for biodiesel has been driven by a desire for energy independence (domestically produced), the increasing cost of petroleum-derived fuels, and an interest in reducing greenhouse gas emissions.Biodiesel can affect engine lubrication (through fuel dilution), as its physical and chemical properties are significantly different from those of petrodiesel. Many risks associated with excessive biodiesel dilution have been identified, yet its actual impact has not been well quantified. In this study, diesel engine oil samples were collected from a fleet of prototype medium-duty trucks with diesel particulate filters running on either ULSD (ultra low sulfur diesel) or B20 (20 vol% biodiesel in ULSD) fuel to determine the effect of biodiesel on engine lubrication. The study consisted of three parts: 1) oil analysis, 2) laboratory friction and wear testing with the vehicle-aged oil samples, and 3) chemical analysis of the wear-protective tribofilms that formed on ferrous substrates during laboratory friction and wear testing. Results showed B20 dilution caused increased lead corrosion, fuel dilution, and oxidation, and decreased viscosity. Laboratory wear test results showed increased wear with vehicle miles for both ULSD and B20 fuels. However, increased wear is not directly related to decreased viscosity resulting from fuel dilution.