Automatic transmission fluid (ATF ), also known as transmission fluid or tranny fluid (colloquial ), is the fluid used in vehicles with self-shifting or
automatic transmissions. It is typically coloured red or green to distinguish it from motor oil and other fluids in the vehicle.
The fluid is optimized for the special requirements of a transmission, such as valve operation, brake band friction, and the torque converter, as well as gear lubrication.
ATF is also used as a hydraulic fluid in some
power-assisted steering systems, as a lubricant in some 4WD transfer cases , and in some modern
manual transmissions .


Modern use

Modern ATF consists of a base oil plus an additive package containing a wide variety of chemical compounds intended to provide the required properties of a particular ATF specification. Most ATFs contain some combination of additives that improve lubricating qualities, such as anti-wear additives, rust and corrosion inhibitors, detergents, dispersants and surfactants (which protect and clean metal surfaces); kinematic viscosity and viscosity index improvers and modifiers, seal swell additives and agents (which extend the rotational speed range and temperature range of the additives' application); anti-foam additives and anti-oxidation compounds to inhibit oxidation and "boil-off" [5] (which extends the life of the additives' application); cold-flow improvers, high-temperature thickeners, gasket conditioners, pour point depressant and petroleum dye. All ATFs contain friction modifiers, except for those ATFs specified for some Ford transmissions and specifically excludes the addition of friction modifiers. [3] According to the same oil distributor, the M2C-33 G specification requires fluids which provide improved shear resistance and oxidation protection, better low-temperature fluidity, better EP (extreme pressure) properties and additional seal tests over and above M2C-33 F quality fluids.
There are many specifications for ATF, such as the General Motors (GM) DEXRON and the Ford
MERCON series, and the vehicle manufacturer will identify the ATF specification appropriate for each vehicle. The vehicle's owner's manual will typically list the ATF specification(s) that are recommended by the manufacturer.

Automatic transmission fluids have many performance-enhancing chemicals added to the fluid to meet the demands of each transmission. Some ATF specifications are open to competing brands, such as the common DEXRON specification, where different manufacturers use different chemicals to meet the same performance specification. These products are sold under license from the OEM responsible for establishing the specification. Some vehicle manufacturers will require "genuine" or Original Equipment Manufacturer (OEM ) ATF. Most ATF formulations are open 3rd party licensing, and certification by the automobile manufacturer.
Each manufacturer has specific ATF requirements. Incorrect transmission fluid may result in transmission malfunction or severe damage.