What kind of fluid do I put into the transmission of a 1988 GMC S-15. I know the transfer case takes transmission fluid, but does the transmission take 80/90W or transmission fluid?

If the vehicle has an Automatic Transmission: Dextron II

If the vehicle has a Manual Trans, its complicated.
Depending on Trans model:
The 77mm 5-speed and 89mm 4-speed overdrive use Dextron II ATF.
The 117mm 4-speed uses API GL5 SAE 80W90 gear lubricant.
GM recommends Synchromesh Trans Fluid (1052931) in both the 85mm 4-speed and 5-speed.
Hope this helps!

I did some investigation and couldn't find what type of fluid for MT, I know there is a drain plug on the lower trans. I would remove it a check the fluid, 90 W has a distict smell, and of coarse auto. trans. fluid is red, and I have heard of motor oil also (smell), I have removed 90W from stand. tran. and added auto trans. fluid, but that was because of cold weather conditions.

Any high quality brand name trans fluid is fine. I have a preference for Quacker State.