The last thing on which every car driver should save up if he wants maximum extend the life of its engine is motor oil. Very important component for safe and smooth operation of any internal combustion engine is a high-quality lubricating of engine parts with oil. Motor oils represent a group of lubricants whose importance is constantly growing with development of motor industry and they constitute the largest part of lubricant consumption. In the beginning for the lubrication were used mineral oils and vegetable oils (castor oil) which are used for racing engines. Due to increasing advancement in regarding the development of the engine, heavier demands on working conditions and longer oil change intervals, started the use of synthetic, and in smaller measures semisynthetic oils for the lubrication. Synthetic and semi-synthetic oils are still in use. Inside the engine large temperature changesoccur. The engine must operate sometimes at a temperature under 0 °C, and on the other side the most favorable temperature in crankcase of oil should be above 80 °C to enable the evaporation of moisture that accumulates as the consequence of the fuel-hydrogen combustion. Temperatures on the crankshaft bearings and connecting rod are about 10 °C above the temperature in crankcase, while the piston rings at higher engine load warm up to 230 °C. For many reasons fuel in Otto and Diesel engines does not burn completely and one part of unburned or partially burned fuel break through into engine casing where it mixes and dissolves in the motor oil. Because of these phenomena and the abovementioned temperature, checking the temperature of the flash point and temperature of fire point of motor oil is extremely important.