My truck has the electronic shift transfer case (RPO is NP1).

The Duramax Supplement book lists DEXRON-III automatic transmission fluid for the "Manual Transfer Case". I am assuming this also applies to the electronic version.

I am thinking about using a synthetic. What specific brand DEXRON III synthetics are some of you using? I read where Mike L. likes to use Mobil 1 for the Allison. Would that be a good choice for the transfer case? If so, is it easy to find? What are some other choices?

I have found a lot of notes discussing synthetics for the front and rear axles, but not much on the transfer case.

Thanks.

I have used synthetic Mobil 1 ATF in past years on transfer cases but it was only because of the extremely low temps where I lived at the time. I have long felt that regular service of transfer case is of more value that hi dollar fluid changed less since it is a closed system with not filter to removed contaminates that are aquired in normal use. Transfer cases are not as strong and overbuilt as they were 30 years ago and fancy fluid does not make them any stronger either. I have had many vehicals go beyond the 200K mark too. I do still use sythetic in a few engines but that is my limit.

I run mobil 1 in my transfercase. it hasnt blown up. Dobt you will find any hard evidence one way or another on whats good not good. Its not as critical beings the additive package dosnt have to be dead on the nuts to keep from slipping stuff or being to "grippy" it also dosnt see the heat. It dont get heated as much as a trans so it breaking down isnt as big of a concern. I think about anything(syn, dino, dog piss) would work in there and you wouldnt notice a difference on way or another. I didnt notice any difference in engaugemnt even cold even though the mobil 1 flows better when cold(then again the TC is ussually 1/2 warm by the time i try to engague it)


but it dosnt take much fluid so you might as well run the best. amsoil is another, redline, just about any fully synthetic fluid from a big co should work the same IMO.Edited by: ratlover

Thanks for the advice. I was able to find two qts of "Mobil 1 Synthetic ATF" at Auto Zone. There wasn't any thing on the front label saying DEXRON_III, but the back label says it "meets DEXRON-III and MERCON requirements".

I also picked up a qt of Castrol conventional DEXRON-III ATF to add to a new spin on Allison filter. The Mobil 1 was 4.99 a qt and the Castrol was 2.69 a qt. Seems like the small difference in price is more than worth the advantages listed above.

I run mobil1 syn atf in the transfer, and mobil1 syn gear in the diffs. Added the mobil1 to the allison filter when I changed it--when I change the ally fluid it will get the mobil1 also. Still on the fence about syn in the Dmax.

When it is 20 to 30 below you can tell a difference. Also, I does use Mobile 1 ATF in my snow plow hydraulic systems and it helps a lot at zero and below. I can tell a difference. Before that, I used to use regular ATF with a few ounces of kereosene added to each quart to help it flow better in cold temps.

I have M1 in my tcase too. After 30K it's still working fine...