When I had my rear rebuilt a while back they added some additive to it. Well i am buying a new diff cover cause the chrome one they put on rusted up and I can't have that. I'm getting a SS one this time. Anyway do you need to add the additive or is that just for break in?

 

The stuff they put in is for the limited slip if your truck has one.

 

^yup Lol which raises the question.....did you ever change the fluid after you broke in the new gears / limited slip? Some oils will already have the additive in it (royal purple, possibly lucas, etc), but others you will need to get the additive.

 

To be on the safe side I would throw a can of the stuff in anyway.

 

I've never had a problem not using FM with LSDs. Most LSDs are too wore out to need it, all it does is make the LSD less effective.

 

He has a brand new aftermarket lsd by yukon i think.

 

If it chatters, add it, if it doesn't it's fine. Most synthetic gear oils have FM anyways.

 

When I made the change to synthetic oil my guy said I could not run synthetic in the rear do to the lsd.

 

Lol I know he put rebuilt, but he ment he had the rear rebuilt (new lsd, new gears, new bearings etc.)

 

He's wrong.
About the only thing I've found so far that doesn't like synthetic is the Powertrax No-slip locker.