My GM 6L90 doesn't have a drain plug, in order to do regular maintenance on it, the pan must be dropped and I usually end up wearing most of the fluid. this allows me to pull most off the fluid out of the pan via the dipstick for a much cleaner service. Since you can't get all of the fluid out of the transmission without having it flushed, I will periodically pull 5-6 quarts of oil from the pan and add new to freshen up the oil quality between filter services