fluid type for A-518

well my transmission builder told me not to run +4 lol, and my old man didnt care for it ether, i remember when it first came out well when valvoline first put it out at my uncles napa parts store that my old man ws the manager of, i said wow this +4 is a synthetic you should try this out in your transmission, and he said no i'll stick to type f, i said whats wrong with the +4 its a synthetic? and he said its not the synthetic part of it that scares me away from it, its all the friction modifyers in it.