Automatic transmission fluid (ATF) is a kind of transmission fluid used in vehicles with self-shifting or automatic transmissions. It is typically colored 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 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.