Front/rear differentail fluids..advice?

I want to change the front and rear differntial fliuds soon, because I don't know when they were changed last and the truck has 91,000 miles on it.

I don't think I have a limited slip differential per my RPO codes. I'm going to go with 75W-90 in both front and rear. Delaware can get pretty cold in the winter, so I'm going with the 75W-90 per the TSB (cold weather ops).

Is it preferable to go with the stuff from the dealer or Mobil1? Also, I'm going to take off the rear differential cover just to take a gander, and replace the gasket.

Any advice?

2000 Silverado pushbutton 4x4, 5.3, extended cab.

when i did mine i put 80w90 in the front diff and 75 in the rear. i only did that cause the manual said to. the front ones a beeyotch to drain/fill, i had to use garden hose pieces.

Get a small flexible funnell and it's a breeze