I wanted to change my front axle fluid on my 2006 2500HD 4+4. My silverado book tells me to use 75w-90 synthetic, and my duramax book says plain 80w-90. You guys know which one is the rite one to use. Also my book does not tell you how much to put in the front and rear axle. I dont know about everybody else book, but mine only tell you how much engine oil goes in thats it nothing else from front to back of both books tell you any fluid caps. So if you could please let me know how much the 06 take rear, and front thanks

3.5-4in back, 2 up front, synthetic 75w90 in both. quarts

4 qts rear. I used GM 'grape juice' which I think is 80w90. Others say any good synthetic like Mobil 1. Front takes 2 qts. I used Mobil 1 75w140.

I used GM 'grape juice' which I think is 80w90.

GM Grape Juice is 75W90 Synthetic

Do you really need to use grape juice in the G80 rear end? Or will regular synthetic work? Mike Z

Regular Synthetic will work fine.

Cool thanks a lot guys for the info.

Mobil-1 75w90 for both, or non-synthetic 80w90 for front only (must stick with synthetic for the LSD rear)

If you do a lot of heavy towing, then it would be wise to go to a 75w140 synth for the rear, but it is more costly. While it will protect better, I suspect that it will break down (lose it's shear stability) more quickly due to the VI improvers needed to bump the grade from 90 to 140.
I'd change more frequently if towing heavily anyways, but the 75w140 would be smart.