What is the name of the fluid used in automatic transmissions?

The fluid used in automatic transmissions is called automatic transmission fluid, commonly abbreviated ATF.

ATF is a specialized hydraulic fluid, lubricant and friction-control medium. It transmits pressure through the valve body, lubricates gears and bearings, helps cool the transmission and controls the engagement of clutch packs and bands. In conventional automatic transmissions, it also works inside the torque converter, which transfers engine power hydraulically.

Modern ATF contains carefully chosen additives, including detergents, anti-wear compounds, oxidation inhibitors, anti-foam agents and friction modifiers. Its color is often red or green so leaks can be distinguished from motor oil, although color alone is not a reliable way to judge fluid condition.

The correct specification matters more than the generic name. Examples include General Motors DEXRON, Ford MERCON and Chrysler ATF+4. CVTs and dual-clutch transmissions frequently require their own specialized fluids. Engine oil, brake fluid and power-steering fluid have different formulations and should not be substituted unless the vehicle manufacturer explicitly permits it.