What type of lubricant is usually used in automatic transmissions?

Automatic Transmission Fluid (ATF) is specifically formulated for use in automatic transmissions, which makes it the correct choice. ATF serves multiple functions within the transmission system: it provides lubrication to the moving parts, helps in cooling the transmission by dissipating heat, and facilitates the hydraulic pressure necessary for shifting gears. The unique properties of ATF, such as its viscosity and additive package, enable it to perform these roles effectively under various conditions present in an automatic transmission.

Using motor oil, gear oil, or hydraulic fluid instead of ATF would not provide the same level of protection and performance. Motor oil is designed for engines and may not withstand the high friction and temperature conditions found in transmissions. Gear oil is typically too thick for the mechanisms in an automatic transmission, which can lead to poor shifting performance. Hydraulic fluid, while it shares some functions with ATF, is not designed for the specific needs of a transmission system and may lack the necessary additives to prevent wear, oxidation, and foam formation.

Thus, the use of Automatic Transmission Fluid (ATF) is essential for the proper operation and longevity of automatic transmissions.