Transmission oil does two very important things.
1. Inside the torque converter it moves the power from your engine to your transmission.
2. It lubricates your transmission.
Your transmission is the second most expensive part of your vehicle to replace. The engine is the most expensive part to replace. Oddly enough, I rarely replace engines, but I replace 6 to 8 transmissions each year.
It is in my professional opinion that you take good care of your transmission, and all you need to do is change the transmission oil.
Years ago there were two types of transmission oils. Now it seems like there is a new one each month. If your car was stuck on the side of the road, it would be okay to use the wrong to get you to the next exit. As soon as possible, though, you need to have a mechanic flush out the old fluid and replace it with correct transmission oil.
The wrong transmission oil, in the short term, may cause your transmission to poorly change gears. Years ago I did a pre-purchase inspection of a Honda. The transmission had a slight hesitation changing gears. Slight, but definitely noticeable. I called my transmission rebuilder who had me check a few items on the transmission. He told me he suspected the transmission had the wrong oil and suggested I remove the old oil refill the transmission with the correct transmission oil. Sure enough, once I changed the oil, the transmission properly changed gears. Luckily for the owner, I was told, the wrong oil would not cause abnormal wear and tear of the transmission.
In 1986, when I opened my repair shop, we changed the transmission oil by removing the transmission pan. When the pan came down, about 3 to 4 quarts of oil came down with it. Often making a big mess of my shirt and my shop floor. Once the pan was removed I had access to the transmission oil filter which I changed before reassembling the pan and adding transmission oil.
Thanks to some mechanical genius, who saved many shirts and shop floors, we no longer change transmission oil this way. I now connect my equipment to a transmission line which runs between the transmission and the radiator. (Transmission oil is pumped to the radiator to cool it. The anti freeze in the radiator and your oil never mix. The transmission fluid has its own set of cooling coils inside the radiator.) Once my equipment is connected to the transmission line, I start your car and the transmission oil is pumped into my equipment rather than the radiator. My equipment pumps an identical amount of oil back into the transmission.
A transmission can hold up to 16 quarts of oil. My newer equipment replaces 99% of the oil in your transmission. The old way only replaced 3 to 4 of the oil in your transmission.
Some transmissions have an external transmission oil filter. These filters screw and unscrews just like an oil filter.
Most manufacturers recommend changing the transmission oil every 36,000 miles or 3 years whichever comes first. A few, very few, like VW recommend that you NEVER change the transmission oil. Your owner’s manual is the best source for information about your car.