Most transmissions from away back did not fail due to fluid failure. But lack of fluid circulation. i.e. dirty filter, valve body and lack of fluid. If you use the newer synthetic fluids, generally speaking you don't need to change the fluid for 100k miles or so. The same used to hold true with rear differentials. 100k miles before oil recommended for change out. Just my two cents. Not into un necessary maintenance for the sake of putting a wrench on something. If I was leaking fluid or was having problems then definitely look into more maintenance. Otherwise, I let it ride!