It is extremely important! Proper gear lube AND break-in is much more critical on a high pinion differential because the gears are NOT submerged in oil like a low pinion would be.
Fill it to the fill plug that is not drilled/reamed out. One of the plugs is made un-useable so you do not use the wrong plug. Different housing designs allow for different amounts of fluid. More fluid always helps cool better.
Best choice (non-racing): https://www.currieenterprises.com/why-run-conventional-gear-oil
OR Torco A248514CE - Torco RGO Racing Gear Oil GL-6
Alternate option: GL-5 rated 85w-140 standard (non-synthetic) gear lube.
For rear applications in desert racing use 2 ½ to 3 gallons of Swepco 203 moly- 250 weight available from SWEPCOUSA.com.
Tighten the load bolt until it's solid against the ring gear, back it off until you can turn the pinion, make sure you can turn the ring gear, with the pinion, all the way around without it touching, back the load bolt off another 1/3 of a turn-2 flats of the Allen head, hold the loadbolt with an Allen wrench and tighten the nut.