I like to run straight 30 weight in mine, but I'm no expert on such things. I just notice that oil pressure seems to decline with heat when I use one of the multi-weight oils.
90 weight 'multi-purpose' gear lube in the trans and the rear axle both.
You will probably need to buy new wheels in addition to tires, so I'd look for an LT type tire that you liked and choose the exact size based on load capacity and the wheels you choose.
The rear brakes on this truck are huge, so you need to make sure your wheel and tire combination will clear them. On the front people often have trouble with the tie-rod ends hitting the tire or wheel when they swap wheels.
A lot of guys look for the 19.5 inch wheels that came out a few years after your truck was made, and run those tubeless wheels with modern tires. I'm told those wheels are hard to find but I've never looked for a set.
