The only additive that OEM's approve is Lubegard. Now having said that, and concurring.... I HAVE.. in a high mileage tranny, knowing it was nearing the end of its life... and KNOWING I WAS GOING TO FLUSH THE FLUID VERY SOON (important)...added Trans-X.
Trans-X and other additives have things in them to soften the seals. I use them do do that, soften... not to leave them in and turn the seals to mush.
After a week or so I then flush the fluid and replace it with new. Maybe add Lubegard at that point if I am so inclined.
I am in the same general mode of thought as Aldive, generally. As a "general" rule, additives are not necessary and can do more harm than good. Clean fresh fluid will do you more good.
(Now you know my dirty little secret though.)
ps. My original never touched A4LD has 224,000 miles on it. It gets fresh fluid often, lubegard and yes (blush) has had a trans-X treatment about 20,000 miles ago.
pps. an additional trans cooler will do your trans a world of good.