What weight oil and how much do I need ? I have ordered new gears and installation kit from Jegs .Rear is 10 bolt ,currently 308 gears .Soon will be 355 behind 700r4. Hopefully this will make it quit lugging so much and help my gas mileage.:yes::yes::yes:

Damn John, that's gonna be a great swap. Let me know how it goes. I just changed the gear oil in mine (mainly for a look-see to find out what gear I really had--3.08:sad::sad::sad:) and I used 80-90. Used less than two quarts, I think I used Castrol. Seems fine. Have fun with the new setup.:yes:

Yeah John...two quarts of 80W90 should do the trick...less than 3 qts for sure with a stock cover.
Are you adding a posi unit? If so, you'll probably need some GM additive.

I used 80W-90 Valvoline with the GM Additive. I used 2 full quarts, plus part of the third quart to assemble and top off the rear.

Not adding posi just want to get from A to B safely not quickly. No racing just cruising. Thanks

What weight oil and how much do I need ? I have ordered new gears and installation kit from Jegs .Rear is 10 bolt ,currently 308 gears .Soon will be 355 behind 700r4. Hopefully this will make it quit lugging so much and help my gas mileage.:yes::yes::yes:

This is where you can use a full on synthetic oil. It runs cooler than standard gear oil and there is no posi unit to worry about. I used Amzoil in all my race car rears.