Motor oil is made from crude oil and is used to lubricate, clean, and cool engines. Types of motor oil include conventional, synthetic, diesel, bio-based, hybrid (blends of conventional and synthetic), and recycled oils. Motor oil varies in weight and viscosity, as well as additives that some manufacturers add to the oil during the refining process. A numerical code system created by the Society of Automotive Engineers (SAE) grades motor oils according to viscosity--the higher the number between 0 and 60, the more viscous (thicker) the oil is.