I need to change front and rear diff fluids in my truck. It as the G80. Is it ok to run Mobil 75w-90 sythetic in the diffs instead of 80W-90? I'm also going to change the fluids front and rear in an 07 Avalanche and 06 Envoy. Is it ok to use that weight in those as well? Do I need to put any additives in the oil before I put it in?