My truck is coming up on 15 K miles & I'm getting ready to do the diff fluid changes. I don't use the 4WD very much - maybe 5 or 10 miles total so far, just to get up some slippery hills in the winter time. Is there really any wear on the front diff at this point - do the gears turn whether 4WD is engaged or not?

 

Yes they turn all the time. I'd just change them out when doing the rear. It's easy and just takes a few minutes + the fluid.

Any 75W90 GL-5 rated synthetic gear oil is good. I use Mobil 1 from Autozone.

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That is correct---change the front diff.. I have Amsoil in mine from one end to the other. Has been that way since the first oil change,tranny,trans case and both diffs along with the larger ATS tranny pan.

 

I only change oil in the front diff every second change of the rear diff. The diff may turn all the time but it is not loaded 98% of the time. I use amsoil 75-90.
Just my opinion
Jim O