I applaud you for being a fluid purist. I agree that is the best philosophy. Normally, I would be very anti-additive. Even good additives, IMO are a waste if the trans and fluid and spec'd properly. But, IMO when we are looking at pushing off an inevitable rebuild, it has its place.
The van in my sig is weird. 1->2 will hiccup and hit hard into gear, like 2nd is delayed and the engine freewheels then bangs into gear under light accel. It's fine if you're getting into it. But 4->3 is awful. It slams so hard that the lights flash and needles bounce to 0 on the dash and I seriously worry about the motor mounts. I added lubeguard, which made all the shifts softer but didn't touch the 4-3 slam. I added 1/2 bottle of lucas, a little at a time, and 4-3 improved greatly. It also firmed up some of the other shifts back closer to normal.If you drove it today, you wouldn't know it was there. The van is our third car, but it's been a year and a half since then and the behavior has been consistent enough that I see it making it a couple of years at this rate before I need to do anything else to it. It will get a rebuilt trans, or just sold. Either way, it was not driveable before, and it is now. Lubegaurd is cherry-red, looks like ATF, pours like molasses.
Good luck