Here to add actual BAD experience with running stupid AT+4 in a 1991.5 Cummins non-lockup A518 OD, it quickly caused shuddering and slipping.
With trans working great about 8 years ago when I bought the truck, I thought I would do preventive maintenance and change the fluid. Dipstick says Dex II or 7176, but Autozone computer says AT+4, so I trust the parts store, plus neither "old" fluid is available, $120 in fluid later (on sale), bought Castrol Transmax Syn AT+4 brand I believe, I did swap about 11 qts.
Within 200 miles its shuddering and slipping. I'm pissed, so I go to Walmart to get cheapest fluid and buy 3 gallons of Supertech Dex III (available then) for $44 total.
I did the full fluid swap, and it was right as rain, perfect, shifts great, not a single issue for years of snow plowing, towing cars on the highway, and didn't ever service it again since that time. 7-8 years later, just retired truck because body is shot, 30 years in NY, planning to swap engine/trans to a better body.
Do not run AT+4, it is too slick. I wouldn't run synthetic either.
Run Dex/Merc and do pan drains/filter often