Reading a number of books , my guess is many trannys will be happy with non detergent engine oil of the correct weight..
The TD in our Navy Launch used 5 quarts of 50wt Non detergent.
Its hard to find so the simple method is to have your NAPA guy order a full case for you . so nothing will be left over that is hard to sell.
Most can do this easily , tho you may have to wait a week or more for delivery as his distributor will have to special order the case.