Why not go to the experts? Allison has a website where you can enter your transmission information and the EXPERTS can tell you exactly what fluid is best for your transmission.
Allison transmissionsAllison recommends several different transmission fluids, made by numerous manufacturers, that all meet their various standards for TES468, TES389, and TES295.
I don't believe that any of the standards specify engine oil or any of the Amsoil products. Perhaps the engineers at Allison know best what works in their transmission and they do not recommend fluids based on advertising and claims by others.