Having worked on off-road Allison transmission's I can tell you they have large Helical gears in there just like manual transmissions do which need extra "shear stability" . Your over the road transmission doesn't have large gears in them. They use Planetary gearsets only. Also that oil must meet SAE standard J300, that's what the "TES439" means. Your comparing apples to oranges here Floridianson .Just to be clear I would never say engine oil peforms better than ATF on road in the Allison. My only fear is I will be the one that with the switch has a problem. I think I will just stay with the engine oil for now in the 915A2. If it has a problem in my life time then since my motor is computer controlled switch out from the governor controlled Allison four speed to a computer controlled newer one. Sold the Deuce A3 so that takes care of that truck. As always your truck so do what you think is best. Was interesting reading that for off road Allison has engine oil listed thanks URSATDX. https://www.allisontransmission.com/parts-service/approved-fluids/off-highway-fluids
The AT1545 needs Dexron to run properly and draining it and refilling it properly will not harm anything.