Mobil1 75W-140 LS ?'s

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Nov 29, 2007
Happy Valley, PA
I was just at PB and got some fluid to change out my diffs. They only had 2 bottles of the 75w140 LS for the rear. I bought 3 bottles of 75W90 LS for the front. Will it hurt to top off the rear with the 90 weight? Also did I get the right stuff? All they had was the LS version my rear is the G80 Eaton locker!

The LS version is fine as its the only type they make. I myself asked the same question when I changed my Front/Rear diffs. If you do not have a LSD in your truck its ok to use, and if you have an LSD you can use it, but DO NOT add LSD additive as it already has it.

As far as mixing the weights especially with differentials i'd taboo it since the differential isn't like an engine but just a bunch of gears and I don't know how it will react or what type of affect it would have. If its maybe 1/2 a quart i dont see the harm but a full quart i'd just go out and buy some 75w140.
If the 140wt is required, then you should go back and buy another bottle for the rearend. You don't want to thin out the gear oil on some of the garbage diff's being shipped these days.
FYI, I emailed Mobil and asked them if the Mobil1 75w90 met all the GM specs for my 2006 YukonXL diffs. They responded "no" and do not use it.
My truck specs a 75W90 but says a 75W140 may be used for trucks that see heavy towing. I think my rear calls for about 2.5qts.
Also, I have read other posts about mobil1 saying there product doesn't meet the spec and it shouldn't be used. However I also read a letter in a post from EATON who stated that using Mobil1 would be fine.
I may just return it for the factory fluid though.

I found a place on ebay that has the factory stuff for 11.99 a qt, and one on craigslist that has it for $5. I don't know if these places are ligit or not though! My local AC DELCO supplier can get it for $20 a qt.
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