Eaton product info says no synthetic but the installer and GM manual say synthetic. What to use? I was planning 80w90 in front and 75w90 in rear, no synthetics. Just got my 4.11s broke in with the new true Trac, changing gear oil Monday.

 

Use regular oil. No synthetic.

 

^Yep. I ordered mine from the man himself, Ron, and he told me to explicitly use conventional.

 

Thanks. I'll get the conventional. BTW, the driveability of this truck skyrocketed with the 4.11 gear and the 6speed trans. It's a shame I had to spend money on this mod and GM couldn't make it this way from the factory.

 

You must use conventional. Some people have used synthetic and it caused the diff not to work properly. I used Valvoline 75W-90.

 

i used redline synthetic with crc stalube posi lube in my stock g80 with 4.10 gears and it worked great.

 

The stock G80 and the TrueTrac are very different though buddy.

 

I use conventional 75w90 lucas on my tru trac. That's what my installer recommended.

 

I have the Detroit Locker... not the Tru Trac but it also says conventional fluid only. I only run Amsoil 75w140 synthetic in it and it works just fine...

 

The Detroit locker doesn't use any clutches or anything that can slip. The gears in the truetrac are designed to work based on friction, therefore they can not use synthetics due to decreased friction.