As far as the gear oil goes, I always changed all the gear oil in the diffs, transfer cases and trannys on all of my trucks and I always used Castrol products. I'm just kinda sold on their products although I do like the Royal Purple gear oil. $17 per quart versus $8 per quart.... for a reason I guess.
This is an oil question, so I'm not really off topic here - ever watched the Castrol as you pour it out of the bottle - when you start it's a nice clear golden brown, keep watching, you'll find that quite often, as you get to the bottom, that nice clear golden brown develops a dark streak.
I no longer use Castrol product - it will sometimes form a sludge -
in an unopened bottle, sitting on the store shelf - do you really want that in your engine/transmission/differential?
Don't take my word for it - when you open your Castrol, look down into the bottle - some will be fine and some will not.