I recently purchased a used 1990 GMC 1/2ton pickup (SL Sierra) with a 5.0L engine and 2wheel drive and am seeking to determine the proper gear oil to use in the 5 speed manual transmission, what do you recommend and why?
Manual Transmission Oil on 1990 GMC 1500 Pickup
The recommended gear oil for your 5spd gear box is something called - GLS; GM part no. 12345349. I have not been able to find an specific information on GLS anywhere. I would suggest to check with your local GM dealer and see what the recommend for your area. The requirements in Alaska may be a bit different.
GM Performance 88900333 Synchromesh Transmission Fluid