The most important function of engine oil is to lubricate. Within the engine there are hundreds of little parts rubbing up against each other. This rubbing creates friction. Friction is the force that resists motion between two bodies in contact. Friction creates heat (thermal energy). Engine oil molecules are like little ball bearings. The oil molecules have a tendency to stick to metal surfaces, but have less of a tendency to stick to each other. Oil decreases resistance and friction between two sliding bodies, resulting in a reduction of engine wear.