Vehicles equipped with automatic transmissions have a lubricating fluid in the transmission that is called, appropriately, transmission fluid. Anyone even vaguely familiar with auto mechanics understands at least that much.
Unlike motor oil, with which trans fluid has much in common, or engine coolant, transmission fluid is usually a bright red color. This separates it from black oil and greenish coolant, allowing the person servicing the car to not only know what fluid they're dealing with, but easily spot leaks and know what their source is.
History, Purpose, and Types of Transmission Fluid
Transmission fluids were first introduced in the 1950s as modified engine oils to go with the (then new) automatic shifting transmissions. Destron (owned by GM) introduced a completely unique automatic transmission fluid (ATF) in 1953 and began the specialty fluid market.
ATF lubricates the trans and provides fluid for use in servos and cylinders. Tranny fluid is both viscous (sticky, slippery, lubricating) and able to be used under pressure. Both are qualities that transmissions require for optimum operation.
So the fluid's primary purposes are to lubricate moving parts within the transmission and provide fluid for pressurized controls within the auto-shifter as well.
Different fluids that are purchased for different uses will have different properties. The heavier-duty trans in a large truck will likely have a much different fluid used in it than a small passenger car will. The one universal additive is dye. The traditional purple or red color comes from the dye added.
Unless it becomes fouled with debris (not uncommon), transmission fluid is technically viable forever. When it becomes fouled with debris from the transmission and its moving parts, however, it begins to circulate those through the transmission, causing problems. This is why most tranny's have filters and magnets for removing those debris. These filters have regular maintenance intervals in the tens of thousands of miles, though.
The other thing that can ruin trans fluid is heat. Most ATFs are OK up to about 200 degrees Fahrenheit. When used at temperatures higher than that, they can begin to lose their viscous properties and at 300 degrees, most will fail entirely. Luckily, most vehicles are made to operate in the 100-150 degree range.
Transmission repair pages