Correct front axle gear oil

[MaverickZ71]

2009 Silverado 1500 Z71 4x4 5.3L with automatic transfer case and 3.42 gears. 

 

Owners manual lists both 80W-90 non-synthetic and 75W-90 synthetic. Which was factory fill? Which would a GM dealer use for a current day drain and fill? 

 

 

[CamGTP]

Rear diff is 75W90 synthetic.

 

I believe the 80W90 was an earlier spec they no longer use, maybe for open diffs. Your truck has a G80 mechanical locker.

[newdude]

Yes, original fill was 80w90 front, 75w90 rear.  GM dropped the 80w90 in the field for the 75w90.

 

So no need to buy two different weights for those year trucks.  You can run 75w90 both front and rear.   

[Jsdirt]

I ran Amsoil Signature Series 75W-110 in both the front and rear diffs of my '07 900-series from 40k miles until I sold it at 134k. Same T-case, but with 3.73's front and rear.

 

The tips of my spider gear teeth were all found in the magnet at 97k, but that's not a lubrication issue - that's a parts quality issue. My G80 stopped working correctly around 23k miles, so there's that, too, lol. 

 

Then the transmission (4L60E) failed 6k miles later ... but I digress! 

Edited May 12 by Jsdirt
additional info ...

[Black02Silverado]

I used AMSOIL's Signature Series 75w-90 in both front and back on my 2002 Silverado with 3.73 gears. Changed it out after the first 500 miles and ran it 100k then changed it again and it had another 100k on it total miles on the truck was over 200k when I sold it.  The UOA I took at 100k on the fluid turned out just fine, the lab said to continue use.  

 

https://www.amsoil.com/p/amsoil-severe-gear-75w-90-100-synthetic-gear-lube-svg/?zo=521390