I can't do anything but add more confusion to this discussion...
I changed the differential out to an 8-pinion on my Bronco 14 (just because I have a phobia of 10-pinions), and it had ATF in it. Prior to 1973, this was what was used in the Sundstrands. I can tell you my transmission was the cleanest, rust free, and wear free of anything like it I have ever seen. The teeth on the gears were sharp-edged and all the bearing surfaces were smooth and quite frankly looked absolutely new. I mic'ed all of those areas and found no apparent wear at all. To this day, that transmission runs faster than any other hydro I have, and has no power issues at all. It also is ready to work from the moment it is started even in freezing temperatures. Of course, the tractor was serviced regularly before I got it, so I'm certain that is a contributor as well.
So, in my opinion- If the tranny has ATF in it, and it works just fine that way, stick with it. The stats on the Hy-Tran Ultra are impressive, but I can't give an opinion there simply because I've never used it. If you do decide to, please keep us all posted on how it works for you. It does appear to have some interesting qualities.
Good luck either way.