6.5 pints, so get two quarts of gear lube.
Once you've filled the differential with lube, be sure to park the truck on an incline with one wheel higher than the pumpkin. Then, turn the truck around and get the opposite wheel higher than the pumpkin. You need to get lube down the axle tubes on both sides. That's the only lube those bearings see. Once you done this, recheck the fluid level and add a bit more if needed.
Heath