Sideways240sx

iTrader: (22)

I searched, but and know what gm suggests but not 100% sure on what i want to do yet.

Im going to install the gm 4.10s in my 98 trans am, m6 this weekend probably, and want to know if i should go with the 75w90 that gm suggests with the 1 bottle of the gm limited slip additive?

Should i go with a synthetic fluid like mobile 1, or royal purple or something?

What would be the benefit of going with the royal purple 85w140 that speed inc suggests? I really have no idea what fluids i need to buy, so any help would be great. Ive never done a gear, but have a mechanic helping me out so shouldn't be to bad. Thanks, Chris

Ps are there any how to's? or things i should know/be aware of before i start the install? Thanks, Chris

keliente

iTrader: (22)

I would go with what GM suggests, and the additive if you have an auburn.

chicane

The benifits ??

Well, lets start off with better temperature stability, lower running temperatures... and about a 50% increase in shockload resistance. The only thing I use 75/90 or straight 90wt in is front ends (4x4) and stock applications. Performance use damn near dictates that you use a better lubericant with better shockloading properties. I dont care if the engineers spent eleventybillion dollars on research and development... and the fact that they put that number in the manual just goes to show you the massive oversight on their behalf. I mean afterall... those same engineers did put a 7.5" mini truck differential into a performance platform that was clearly underrated to begin with.

On the subject of lubericant... I however, would not use a full synthetic. In fact if it were up to me, I would use a straight weight 140 that is parrifin based. The Auburn will also benifit greatly from its use as well.

Sideways240sx

iTrader: (22)

how do i know if the car has an auburn? im trying to look in the faq, but dont see anything. Thanks, Chris

chicane

Well... if the differential has a posi in it... it might be an OE Auburn.

Really, the only way to tell exactly what you do have... is to pull the cover.

alamantia

iTrader: (10)

you dont have to pull the cover if you jack the back of the car up

https://ls1tech.com/forums/gears-axles/747040-did-everyone-get-stock-posi.html

dailydriver

Unless you, or the previous owner put something else in there, you have an Auburn. I most definitely would NOT use synthetic gear oil with an Auburn diff!

Grn4fbody

iTrader: (2)

In ANY rear end i use Lucus flood, we use it in the Semi's at work and all, but you do need the additive for the posi

chicane

I dont run it straight... but as an additive its great.

...and yes, you will still need the friction modifier.

dailydriver

chicane; I believe he's referring to the actual Lucas mineral based gear oil, like I'm using right now (in 85W-140). It's also one of the "default" gear oil choices you've suggested in the past, in a pinch, if one cannot get Shaeffer's 209 or Torco RGO soon enough for their change.

Grn4fbody

iTrader: (2)

Sorry i cant type and or spell