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?

[Stocker]

I heard years ago that you can either use 80W-90 gear oil, or ATF in NP205 transfer cases. My Clymer manual calls for Dexron II ATF (I believe type F is Ford). I have Mobil 1 75W-90 in my NP205, which I put in before I got the manual. It's synthetic and hasn't caused any problems with the seals. I would use any good brand synthetic 75W-90.

[Rubble]

From what I been told,ATF is too light for the 205,I have used 80-90 for the past 20 years in my 1980, never had an issue even with -30C temps. 80-90 for 205,ATF for 208
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[V30 mongo]

I have a 1988 V30, SM465/NP205 since new. It hat a PTO unit installed when new. At that time it was filled with ATF from the factory. After the PTO installation 80W90 GL5 was used. The truck still runs now with no transfer case problems and only regular fluid changes.

[chevak20]

Thank you everyone for the suggestions. They are all helpful and what I suspected, just wasn't sure before. Thanks.

[ryanroo]

205s are pretty indifferent. put oil in it and it will survive. in many trucks for many years we've run 80-90. i like synthetic but it all works.

ryan

[Dooley]

My 1972 called for 80/90 gear oil in the case
My 1980 called for 30w motor oil in the case
My 1988 calls for ATF in the case.
They all work in a 205.

[barraza]

Mine started on gear oil, but runs fine on dextron leaked past the transmission to transfer case seals. A 205 seems to run fine on just about anything.