The factory service manual for my 99 Ram Van calls for Mopar 80W-90 gear oil in the differential, but my local dealership's computer says it should be 75W-90. So I have 2 questions:

1. Which oil weight do you think I should go with, or do you think it matters?

2. Does brand matter? I don't really mind spending twice as much for the Mopar gear oil if it's somehow better/different, but I'll have to order it, whereas I can get something like Mobil 1 locally.

 

You can likely use either, and not worry about it. Also, in my personal opinion, brand really doesn't matter. So long as it meets the requirements for whatever grade it is supposed to be, it's all good. Just need to know if you have the limited slip rear diff or not, so you know whether you need the friction modifier additive as well.

 

As long as you're close in weight, you'll be fine. Now, if you put 90W-140, your performance will suffer. I've only used that to quiet down extremely worn differentials pending repair.

 

The sticker under my hood says I have a Chrysler 9.2 rear axle, but doesn't mention limited slip or Trac-Loc. I had the dealer look up my VIN, and their computer also said I don't have limited slip.

Is there anything else I should be looking at to confirm that I don't have limited slip? Should I be able to tell by looking, once I open up the differential?

 

Yeah, once you have the cover off, it'll be obvious if its an open diff, or some variety of limited slip.

 

Just jack one wheel off the ground and put the truck in neutral. Chock the wheels of course. If it's open, you can rotate the wheel. If it's a sur-grip, you can't.

 

Brand doesn't really matter, I put 85w140 in all of my axles regardless of what it calls for.

 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ME TOO ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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