Laboratory Analyses of Sample Oil Evaluate Oil Viscosity, Cam Wear, Acid Number, and Other Characteristics: In March 2007, the law was amended in Japan to allow operation of diesel vehicles on biodiesel fuel containing biofuel up to 5 percent (B5). The question was then raised about the deterioration of engine lubricating oil used in diesel engines operating on B5. Thus, a group of researchers from auto and oil industries and trade organizations conducted investigations to characterize the oxidation stability through laboratory analyses of engine oil sampled in the field. Oil was sampled from various commercial vehicles, e.g., trucks and buses, operated for about 30,000 km on B5 containing ester as high as 3 volume percent. A 3.907-liter, inline four-cylinder, turbocharged direct-injection diesel engine was operated on various B5 fuels in addition to diesel fuel on an engine dynamometer for observation of changes in oxidation stability. This chapter reports field test results as well as laboratory test results of various engine oils contaminated with various B5 fuels and diesel fuel.