I have searched but can't seem to find a clear cut answer to my questions. If i buy syntetic mobil 1 for my rear diff it says it will work in limited slip, so should I still put the additive in there or would that mess things up. Seems to me like it would be too much and might causea proble.Also for the front diff can I use syntetic?

 

http://www.royalpurple.com/prodsi/mgeari.html
Best way oil to use. I recomend it and prefer it over Mobil 1 synthetic

 

You do not need to add any additives for this one and yes, you can use this in the front differential

 

Some limited slips dont like synthetic though, my auburn posi is a picky eater and hates synthetic. If it ha sthe additive that's enough then.

 

DC spec's synthethic 75W140 for the corporate 9.25" rears on the 1500s. M1's 75W140 contains LSD additive, their 75W90 does not. Valvoline, Royal Purple, and Amsoil, to name the one's I've personally used, all contain the LSD additive. My order of preference is: Amsoil, RP, Valvoline Full Syn, then M1.

BTW, DC spec's 75W90 dino or syn for the front diff. I've heard several folks use 75W90 in the rear if there's no towing or carrying heavy loads for long distances. Interestingly, the 2500/3500 rears spec 75W90 syn.

 

I use the oil that my posi specifies as I want to keep it in working condition and that's 80w90 non synthetic.

 

BTW, what change interval does Auburn recommend? Is it still 15k miles?

 

Yes I believe it is the 15K. I just did my 500 mile change on my new gears and I'm going to change it at 1000 as well then do the 15K's after that.

 

^^^ sounds like your breaking them gears in good. they should last a lifetime

 

so what weight is the best for dodge rearends?