Which rear end fluid should I run?

[Luis]

I tow during the summer but not that much in the winter. Is it O.K. to run the 75w 140 year round? Will it hurt if I do? Or is the 75w 90 O.K. year round?

[LittleBigRig]

Luis,

I've posted the "75-140 Synthetic Rear Diff Lube" thread to try to figure out what the "official" word is from GM regarding the heavier weight gear dope.

Up to this point, all I can find are third-party suggestions - but nothing official like an owners manual recommendation. If no one can offer up an official word, then I have to assume that GM is quite happy warrantying the GCWR of 20,000lbs+ on 75W-90.

[JimWilson]

Since GM actually states that it can be used I don't think there should be a problem. But it does get kinda cold in MD when winter rolls around, so unless you tow something very heavy, and you do so often, it might not be worth it.

[mark45678]

I ran 75-140 in my 01 and found no benifit, infact I think it hurts fuel economy slighly.... I changed my 04 to mobil 1 75-90 with only 1200 mile on the truck both the rear end and t case had LOTS of fine break in fileings!!!!!

[bob]

why not run the 75/90 synthetic and install a mag-hi-tec cover which adds a few extra quarts for cooling in the summer and keeps the lower weight for the winter,plus it has a drain plug so you can keep it fresh easier!

[Lone Eagle]

The OEM axle has a drain. It is also the toughest rear end in any of the new trucks. I can't remember reading of any problems with it. Later! Frank