what type of gear lube to use in my 2006 gmc, any GM techs out there

[mc1984ss]

I want to change the gear lube in the back of my 2006 GMC truck with a limited slip. The factory manual says to use a synthetic 75w-90 gm part no. 12378261 meeting gm spec # 9986115

I have been doing alot of searching and cannot find a gear lube that matches this spec and the gm stuff is very very expensive! I have read alot of forums and almost everyone is using mobil 1. It says it is fine for most limited slip diffs but Mobil 1 does not meet this gm spec. Supposedly the stock stuff looks and smells like grape juice. I dont want to dump just anything in there I want to dump in the proper stuff I am just trying to avoid the high cost of gear lube at supposedly $30 a QT Any help is greatly appreciated!

[daved931]

I want to change the gear lube in the back of my 2006 GMC truck with a limited slip. The factory manual says to use a synthetic 75w-90 gm part no. 12378261 meeting gm spec # 9986115

I have been doing alot of searching and cannot find a gear lube that matches this spec and the gm stuff is very very expensive! I have read alot of forums and almost everyone is using mobil 1. It says it is fine for most limited slip diffs but Mobil 1 does not meet this gm spec. Supposedly the stock stuff looks and smells like grape juice. I dont want to dump just anything in there I want to dump in the proper stuff I am just trying to avoid the high cost of gear lube at supposedly $30 a QT Any help is greatly appreciated!

The fullsize trucks with RPO G80 actually have an Eaton locker. You don't need any additive for those units.

I would just use a good synthetic gear oil like Royal Purple or Mobil 1.

[ss427]

I can't believe that suddenly rear ends are made so "special" that they have to have a special lube. WTH, most any
diff. lasts hundreds of thousands of miles. With 1 fluid change in its life, generally they last forever. Never even
thought about putting synthetic in any of 'em.

[TC]

I'd go with Royal Purple, even if you need the posi additive Royal Purple already has it in it......

[daved931]

I can't believe that suddenly rear ends are made so "special" that they have to have a special lube. WTH, most any
diff. lasts hundreds of thousands of miles. With 1 fluid change in its life, generally they last forever. Never even
thought about putting synthetic in any of 'em.

Usually the only thing "special" is that they are synthetic. The manufacturer just has their logo put on it and they mark up accordingly.

GM AutoTrak fluid is blue, but it's really just synthetic ATF.