fluid type for A-518

well the guy who re-built my transmission said no synthetic atfs only dexronIII or atf+3 and he said whoever runs type f in a torqueflight is an idiot.... I hve to strongly disagree with his statement on that subject, the most intelegent mechanic and mopar guy i knew ran type f in the 727 and nothing else and i know for a fact that he was not an idot.
I dont know why he says no to running a synthetic? i said well +4 is a synthetic and thats what dodge says to use, and he said but you have a non lock up converter +4 is so highly friction modyfied that you do not need it, he said you could get away with using +4, but your transmission would not be performing at its best potential on it.
He said dexronIII or +3, preferably dexronIII is has less friction modyfiers then +3, i said what about dexronVI and he said no its a synthetic like it was a bad thing? it pretty much made it sound like running a synthetic is a bad idea same with type f so idk.