Never really thought about there being more than one type of 90 W gear lube. Always used the same stuff in transmissions and rear ends in the past. My understanding is that GL5 is for rear ends and GL4 is for manual transmissions like my toploader. I know the modern 5 and 6 speeds use ATF or some similar oil, not 90W.  There is some question as to how well Synthetic will work. Synthetic is rated GL5. I can find non synthetic rated GL5 , GL4. Also I see both 75w-90 and 80w-140 recommended. What are you using and why? I have a quantity of 80w-140 rated GL5, GL4 on hand. Transmission has been rebuilt. Plan to change the oil after 500 miles or so. Will GL5 cause problems in the transmission?   While on the subject, is Synthetic GL5 what I want in my 9" rear end? I know I need the Ford additive if I have Traction Loc.