Thanks for clearing that up TeW.
I know it's not GMC specific, but this
1937 Chevrolet Owners Manual provides the back then lubrication requirements/specs that should be equivalent to your truck.
The one thing to acknowledge is that today's lubricants have different ratings/descriptions than those back in the day. I'll not try to describe all the ins and outs of that as there are already a lot of discussions on the topic, mostly in the
Driveline forum.
Hopefully others will chime in here with some sage advice.
