If you were to read owners manuals from various manufacturers and various years, you'll find that the recomendations have changed several times. I've tried several types and noticed zero difference. On one machine I tried running 2 different fluids back to back. I noticed when changing fluid type that the trans was really grabby, it took a few more fluid changes to get rid of this feel. I would stick with what has been working for you. On a freshly rebuilt T-20, I take an air engraver and mark the case right above the fill hole with "ATF ONLY!" then under that the type of fluid that I used. I wish the T-20 had come from the factory with "ATF ONLY" cast into the case or on a tag near the fill hole. I've seen too many with gear oil or motor oil in them!
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.