moore_51

I have a 98 gmc truck with a 10 bolt that I'm putting a Detroit locker in with 3.90's. Wondering what weight oil I should use. Only ever sees the street. Will at some point start seeing the track some but not a whole lot. It is my play truck so it's not by any means a daily, so I do beat on it a bit on the street. The only thing I know for sure is that it wants a petroleum/mineral based oil. Just don't know what viscosity to use.

SteveC

I would use straight 140W dino oil.

BOLO

iTrader: (40)

I use Lucas gear oil- 85W140

1 FMF

iTrader: (1)

you choose viscosity based largely on temperature. if your in a hot climate and if the application was more on the severe duty side of things like heavy vehicle, towing, going up hills, etc. which would lead to higher oil temps then go for the ##W-140 gear oil.

if track means a pass that is under 20 seconds, then you would be better off with 75w90 oil. higher viscosity oil does not always lubricate and protect better.
you only have the 2 choices for viscosity and neither is a wrong choice.

i believe its the truetrac and the helical gear style diffs that prefer the non synthetic gear oil, and that is prefer not require. the locker if you look up some drawings and animations of how it works would not be affected by synthetic gear oils. what causes the need for a gear oil and viscosity requirement is the ring and pinion. your best choice might be a synthetic 75w-140 oil.

99SSJarhead

iTrader: (2)

It is not so much that they prefer dino over synthetic, its that you are more likely to find dino without friction modifier additives which is what those types don't like. Moser is now recommending a sysnthetic brand (Motul 300) for their ***trac brands (and most everything else) because it is additive free. Not sure the likelihood of finding it at O'Rielly's or NAPA though so it'd be an online purchase. As such dino oil is still the best as it is easier come by locally.