Automatic transmission fluid (ATF) 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. On most vehicles its level is checked by a dipstick while the engine is running.
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.