fluid type for A-518

what could s synthetic possibly do to my transmission to make it a bad idea? i cant think of anything, all i can think of is the better things it would do for it. He seemed to think dexronIII would be the best possible choice, so if he says dexronIII why wouldnt dexronVI work just as good or better? they say dexronVI can be used in anything calling for dexeronIII just like +4 can be used in anything calling for +3, and dexronIII was factory fill in my transmission back in 1991 so therefore dexronVI should be able to be used.