Disorders of operation machines, are caused by a whole group of internal and external influences and processes that act and under way directly in machines. These influences have the effect of changing the properties of machine parts. A summary of these processes is called a mechanism of failures. This mechanism failures and their effect on the properties of machine parts is changed with new technologies, which include the use of biofuels. As the most widely used substitute of fossil fuels, which covers the majority of the energy consumption in the transport sector, particularly in the automotive industry, is in currently enforced ethanol biofuel E85. The European Union is heading to efforts on promote the use of biofuels. Biofuels are substitute's fossil fuels only partially. They are produced by blending fossil fuels and biofuels. Effect of using E85 was studied on the type of passenger car Saab 9-5, engine B235R. From the car is taken sampled of engine oil during operation on E85 and during operation of fossil fuel BA95. The internal combustion engine in the analyzed vehicle uses motor oil of type Mobil 1 0W-40. From analyzes of samples is confirmed a great influence E85 on the viscosity of motor oil and therefore also to the deterioration of lubricity and increased engine wear. Measurements were performed in the laboratory of quality and reliability of the machines at CULS in 2014 and the first half of 2015.