[hatzie]

I just did a new-to-me fluid swap on my 2005. The transfer case oil ran out pretty dark. The axle fluids didn't look much better.

The transmission and engine oil looked pretty good but the oil pressure runs around 35psi and rises with RPMs with the new WIX filter and 5W30 Synthetic I poured in the 6.0... It didn't on the old oil and filter. The 4L80E is unchanged on the fresh pan full of Dexron VI with a brand new strainer even tho the oil wasn't clear like red glass.
GM released TSB 08-07-30-040C about pan magnets that were not up to the task on the 4T65E & 4L60E. I replaced the pan magnet on my 4L80E with the one from the TSB. The donut magnet in the TSB is supposedly the same part as the magnet that's supposed to be present on top of your M74 Allison 1000 spin on filter and usually isn't. The Dirtymax diesel guys don't like to use anything but the Allison filters because of the significant differences in the way the aftermarket filters are constructed.

I changed all of the fluids at 83,000 miles on the clock so it might've been the original juice from 2005 in that xfer case or it might've been some unknown pedigree fluid put in by the previous owner.
I ran it for 2,000 miles and changed em again. It ran out of the transfer case and axles just a little darker than the new stuff so I'm pretty much good to go.

If you have the Eaton G80 Locker in your rear axle DO NOT use limited slip additive. Some Gear lubes have Limited Slip Additive already in the witches brew so I'd look carefully at the bottle.
See TSB 91-4-109

I'll use Delco fluids where they make monetary sense but I'm not a fool... The Delco 10-4016 Gear oil I just used in my 2005 has gone up in price drastically. I only paid $11/qt... about on-par with Mobil 1. It's almost $18/qt from Summit now and I cringe at the thought of what a GM dealer would charge for it. At those prices I could buy designer oil from Amsoil or the like.
Honestly any decent quality 75W90 Synthetic GL5 gear oil will work in both axles as long as it doesn't have Limited Slip additive. $18 is too high for the pretty Delco label when I can get Mobil 1 or Valvoline or Castrol or Pennzoil 75W90 GL5 Synthetic for a much more reasonable price. I don't bother with store and other off brands but the big names in oil are all pretty good.