Mine has the drain plug too, and well as the overflow tank to add fluid. I could be wrong (I often am) but my thinking is if I changed the fluid without changing the filter...would it help or not. The only way to change the filter on mine is to crack the case....When I got her, I immediately changed the transmission fluid. It was BLACK (mine has a drain plug!) I've changed it again twice. The last time I changed it because the rubber hose to the plastic oil level viewer had worked loose and that transmission was down 2 1/2 quarts before it quit! I drained out the rest, filled her up, and today she's perfect!!!