Putting transmission fluid in a car's gas tank can lead to zinc or phosphorus compounds contaminating the oxygen sensors. Effects vary based on the car and the amount of transmission fluid in the gas tank.
Oil or transmission fluid was sometimes added to gas tanks to clean and lubricate the engine's valve train. However, this practice became unnecessary around the 1960s due to valve guides becoming more resistant to wear. Gasoline also contains detergents which clean the engine more effectively than oil or transmission fluid.
The fluid used in a transmission varies based on the car. Cars with an automatic transmission use automatic transmission fluid, whereas cars with a manual transmission may use regular motor oil, heavyweight hypoid motor oil or automatic transmission fluid. Transmission fluid lubricates a transmission's moving parts. In an automatic transmission, it also acts as a coolant.
The transmission fluid should be changed according to the manufacturer's recommended schedule. If the fluid isn't changed often enough, contaminants accumulate in the fluid. This reduces the transmission's performance and lifespan.
The method used to change transmission fluid varies based on the repair shop. The owner's manual has the correct method for changing the transmission fluid, and in certain cases recommends a specific type of fluid.