So I recently had GM 4.10 put in my 2002 ws6. The shop that did the swap for me used Valvoline 80/90 lubricant. After looking in my owners manual, I see that it suggests I use a SAE 75W-90 Synthetic axle lubricant since it's a limited-slip differential. Am I going to run into trouble or have problem since they didn't use synthetic? I was also about to change lubricant after my 500 miles break in period so I'm curious if I should stick with the 80/90 or go over to a SAE 75W-90 synthetic and add the 4 ounces of limited-slip axle lube additive it suggest I use during a complete drain and refill.

 

As you already said in another thread I seen you were about to change it. How many miles are you from changing it now? The 80/90 and 75/90 isn't really a big deal but the limited slip is. If it calls for it then it needs it. I know in the royal purple diff. oil the limited slip is already in it.