1999 Ford Ranger Mixing fluids in error

If you put motor oil into the transmission and ran it you should drain all the tranny fluid out and put fresh fluid in twice ( run it between changes a 1 mile or less). The reason for doing it twice is because the motor oil most likely went into the torque converter. If you did not run it you can just drain the fluid out and replace it with fresh tranny fluid.  Transmission fluid differs from motor oil in one important aspect --tranny fluid expands when hot , motor oil does not. Motor oil also breaks down at a lesser temperature than tranny fluid. If you leave motor oil in your tranny it will eventually destroy the tranny....BTW when you drain the tranny fluid from the pan there is still about 3 quarts in the torque converter.