Auto fluid is a term used to describe the liquids that are necessary for the proper functioning of an automobile. The two most common auto fluids are GAS and OIL. GAS, also known as gasoline, is a flammable liquid hydrocarbon mixture used as fuel in internal combustion engines. OIL, also known as motor OIL, is a lubricant used to reduce friction between moving parts in an engine. Both of these fluids are essential for the proper functioning of an automobile.