Engine oil is used in vehicles. But many people do not know its use. Consider an object kept on another object. When it is drawn fiercely or pushed it aside or rubbed with the other object then friction is produced between the two surfaces. This frictional force always opposes the motion of the body. That’s why ball bearings are used in the wheels of bicycles and motor cycles for free movement. Friction is produced due to movement of the rolling metallic balls. That’s why it is easy to roll any object. In engines or moving wheels, friction is produced due to the rapid movement between the two heads of pistons. Due to that friction, the capacity and efficiency of the engine decreases. To reduce the friction, a grease or oil like jelly liquid substance is used as fuel in machines, engines and moving wheels. Due to the thick oil, a thin layer is formed in the movement of piston. This layer of oil reduces the friction. Due to it, capacity and efficiency of the engine do not get reduced and engine’s use can be prolonged for more life.