np 205 oil type dilemma

[chevak20]

Too much to choose from nowadays.
Atf type F fluid, synthetic 75-140 hypoid GL 5 or 80-90 hypoid GL 5?
The th400 has dexron III in it and the gear vendors old type J behind the t-case requires aft type F.
I Googled np 205 oil type and some people use 80/90 GL 5 in summer/or all year around, aft fluid in the winter but none refer to it as type F, as this fluid I think, would be most suitable as it has been used in other gear applications like volvo's m46 manual transmissions.
My np 205 is an -86 model and it has a red atf kind in it now and I want to change it- But to what? Any suggestions?
I might consider 80-90 GL 5 hypoid all year around as it usually never drops below 5° F.
Does synthetic oil do any harm to old seals?