Can't speak for Terry, but I do know that in later recommendations he sticks to the Ford fluid, and only mentions synthetic diesel oil if you have no other options.
Amsoil themselves recommend their 5w-40 for that application.
That quote is several years old...perhaps formulations have changed? I do know the Ford stuff hasn't changed, and its still an ideal fluid for the MTX-75. I have heard good and bad stories with just about everything else, Redline, RP, Amsoil, etc. In fact, the only other fluid that I heard solid positive feedback on was Specialty Formulations MTL-P, but good luck finding that.
Either way, what works best is what works best for you. So use what you have, and if its satisfactory...keep it. Word of advice however...depending on how old your current fluid is, anything fresh may be an upgrade. Ford lists the factory fill as lifetime on the trans, but I have noticed that after about 60k the shift quality is a bit off. I have 30k on my latest fill, and things are still great.
Cheers!