rear diff fluid change

[mark5740]

What size hex head plugs are on the diff unit?? It says in the manuel to use 75w-90 syn gear oil. Is that right??

[Gray]

Welcome to the forum Mark.

Sorry I don't know the hex size of the plugs.

The manual says 75W90 synthetic, but what it doesn't say is that these trucks don't have limited slip diffs, so if you want the most fuel efficient fluid look for 75W90 for 'non limited slip diff' applications, ie: a more slippery oil with a really low friction coefficient, too slippery for limited slip diffs. I've been using Motul ester based 75W90,...I think Redline makes another ester based 75W90 for non-limit slip diffs as well and there should be a few others.

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[Gray]

Here's a VOA (virgin oil analysis) of the ester oil based Motul Gear 300 75W90 (not for limited slip diffs) that I had sitting around in the shop for a few months, and which have been in both front and rear diffs for about 25K kms. I've looked at VOA anti-wear and detergent metal numbers of other gear oils listed as for 'severe service' and nothing compares to this brand that I've seen so far.



Here's a used oil analysis UOA of a Mobil synthetic 75W-90 for comparision;

http://www.ezdries.net/Vidpics/temp/4R% ... 0share.pdf

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[LittleStevie]

The hex size is 10mm. I went with Royal Purple MaxGear 75W-90, which meets the GL-5 spec. I believe that it does have friction modifiers for use in limited slip differentials, but I doubt you'd notice much difference in economy.

Make sure to get 2 new crush washers (one for fill, one for drain) before you start the job.