Automatic transmission fluid, or ATF, is used for lubrication, warming, cooling, hydraulic actuation and turbine flow, and a bath for the friction material and the electronics.
The fluid has a hard life, but with regular servicing, the transmission can last for many thousands of miles with few issues.
Manual transmissions use gear oil, which is much heavier than ATF. Gear oil uses a friction-modified formula to provide good cold-flow properties to promote smooth, fast engagement of synchronizers and gears for smooth shifting.
This video is sponsored by The Pronto Network.