Definitely use GL4 and not GL5. NAPA has GL4 dino oil in 2 qt jugs...that's about the cheapest I've seen it. It's typically not stocked at most other chain auto parts stores.

Or, you can do like I did and go with the NV4500 full synthetic Castrol used in 1ton Dodge and Chevy trucks. Costs about $90 for the 6 quarts, but you get the piece of mind of a full synthetic, and best of all, the stuff is odorless, unlike most other stinky gear oil.