Ill add my .02 for whatever its worth.
I do not believe in additives, engine or transmission. When you buy oil/atf etc there is already an additive package in the oil, anti foam, anti wear etc, that they have balanced to what is needed. When you add other stuff you throw that balance off. I have seen a test that someone did with the lucas engine treatment, and holy crap you should see how much that stuff foams. For those that arent the most savvy with oils and stuff, foam is air bubbles, and air bubbles means there is no oil there. So when that runs through your bearings, gears etc, there is no lubrication in that air bubble. This will cause quick catastrophic failure.
The only time i support the use of additives is if it is for sure dying and you just need to buy some time. Like for me, i've had an engine with a knock, clearly its about to kick the bucket, so i dumped lucas in it which quieted the knock down for a while. Just bought time is all.
I feel the same about transmission additives, if its got serious problems and you plan on a rebuild or new soon, use the additive to buy time. But NEVER put additives in a decent functioning part.