I am stepping down from 4.10 to 3.73.(New) and POSI (SLP Take Off)
I am wondering is it good idea to use Reg Gear oil for 500Miles.
Then change it to Mobil Synthetic gear oil with GM additive.
Or just not to bother with it. Just put Good ol Mobil and GM add ?
anyi deas ?

 

I don't think your idea is a bad one. Since the gears are new, it is reasonable to expect a little "breakin". Changing the diff fluid after 500 miles or so will get rid of any debris. I would recommend the synthetic at that point. Some Redline 75W-90NS should do the trick, and you won't need the GM additive.

 

Bingo
that is what I wanted it. I wanted use Mobil one.
Also I have Griddle non stock so I have no magnet

 

I'd change it again at about 7,000 mi

 

Screw that. USe synthetic from the get-go and make sure you use the additive. I did a 3.73 swap for my 2.73's and used a T/A girdle, all synthetic, SLP takeoff posi. I never have to worry about that rear end fluid.

 

Why so early again ?

 

I just want to extend the life.
Why not
Its only cost 2 new bottle of Mobil one Gear oil

 

I broke mine in with regular gear oil for 1000 miles and then put in Mobil syn and the GM additive.
I just changed it recently and the gears looked great.
I know they say you don,t have to use the additive if you use syn but I put some in anyway, it can,t hurt.

 

I think I am gonna do same

 

My engine builder pointed out that Eaton recommends not using sythetic but I am anyway. I have had no issues that I am aware of.

 

Actually Eaton and Auburn posi recomend not; cause they use clutch, Synthetic oil screw ups those