Fluid Change In '06 Envoy

[Mossyoakglock]

I am going to be changing the front and rear diff fluid in my sister's 06 Envoy. I don't have a clue what rearend, etc is in the Envoy. How do I find out? I am also going to be changing the fluids in my dad's 07 Avalanche 4x4. I am pretty sure he has the G80 and will eventually be changing both front and rear fluids in my 08 Silverado w/ G80. With both the 07 and 08, I read that it's ok to go with 75W-90. I'll most likely go with Mobil1, but what about Valvoline Synthetic? Any good? With Mobil1 or Valvoline, do I need to and any additives to the oil when I put it in?

 

And back to my original question...Is it ok to use 75W-90 in my sister's Envoy, both front and rear? Do I need to add any additives to the gear oil?