reidry

Called to check the availability of the highly recommended Mobil 1 Synthetic gear lube for my differential and was asked if I wanted 75w90 or 75w140. I'm getting the GM posi-additive from the dealer, but need a recommendation on the viscosity of the gear lube.

Appears that factory spec was 80w or 80w90.

What's the recommended viscosity in the Mobil 1 Synthetic?

Thanks in advance,

Ryan

mothersworry

I just replaced mine with 75w90 and two bottles of the posi fluid.

scorp508

70w90

The 90 is what it'll be when warmed up and thats closest to the stock spec.

rws.1

I agree witnthe last post . Go with the 75W90 the 75W140 will be too viscous.

larryfs

do you need the additive with the synthetic gear oil ??

Tom Piper

I haven't confirmed this, but I read a post that stated GM knows (and maybe even issued a service bulletin on it) there is a problem with synthetic gear oil and the silicone sealer that seals the differential housing.
The post said something about the synthetic gear oil deteriorates the silicone sealer and causes leaks.

I do know that I meticulously sealed my Dana 44; and, after about 6 months, it leaked with synthetic Mobile 1.
I, then, took it apart and resealed it and now use standard gear oil with no problem.




I also had Mobile 1 gear oil in my rebuilt '64 coupe Muncie transmission. That car is seldom used.

You're not going to believe this one!!!
After about two years with the Moblie 1 in the transmission, I noticed the case was wet on the outside. It looked like someone lowered the transmission into a bucket of gear oil up to the filll-plug on the case. The gear oil was perfectly clear. My guess is the aluminum case was porous enough to allow the synthetic gear oil to seep through the case.
I replaced the Mobile 1 gear oil with standard gear oil and haven't seen a problem yet. However, if it took two years for the synthetic to seep through the case, there is probably still some seeping through now.

Tom Piper

reidry

I have seen discussion that leads me to believe that the synthetics will find leaks where regular dino oils will not.

Would like to hear more experiences from other members as I intend to reseal, fill and reinstall my D44 this weekend.

Ryan

Tom Piper

Go to this web page:
http://www.alldata.com/TSB/10/92100604.html

and, look at item number 48:
76-02-02A OCT 98 Drivetrain - Recommended Axle Lubricant

If someone has access to the complete alldata technical service bulletins, maybe they can look up this one.

Tom Piper

DANSURE1

I Have a 71 vet LS6-425 hp. 90 vet never use anything but dino oil,89 chev p.u. MINT-280.000 MILES HARD MILES RUN GREAT.I'm new here but i do no alot about oil, synthetic 95 percent of which is made up of one base chemical (typically ethylene glycol).I can go on all night but i will say , Premium conventional oil is the standard new car oil. All leading brands have one for service level- SL. I do H.V.A.C. WORK BUT I HAVE BEEN WORKING ON ENGINES FOR 30 YEARS.

silver & red CE

Sealed my diff with Petex Right Stuff and filled it with Mobil1 75W-90 with the same rating as the factory requires (is it G?). I do add one bottle of the GM factory friction modifiers.

No leaks or issues, about 9K miles of use with many 1/4 miles runs.

I run every synthetic fluid I can. Mobil1 engine oil, Castrol tranny lube, Mobil1 rear diff, Mobil1 chassis lube. It's proven to run cooler. I'm swapping my valve springs and seals, after 30K miles my engine is clean as a whistle, I attribute this to Mobil1.

Eric

Tom Piper

For the LT1, Mobile 1 engine oil was factory fill and is required to maintain the warranty.
However, I am not aware of any manufacturer that put synthetic gear oil in the differential from the factory -- I wonder why?

And, I still want to know about the GM "techinical service bulletin" (#48 listed above).
The post I read several months ago stated that GM was of the opinion that synthetic gear oil would cause the original sealer that GM used on the differential to deteriorate (this means it would actually attack the sealer and the sealer would decompose, not just happen to leak by it).

And, in my case, there is no doubt (I am known for over-kill, I actually degrease the surfaces before applying the sealer) that I sealed my C4 Dana 44 well; and, in about 6 months, it was leaking with Moblie 1 gear oil in it. I re-sealed it and put standard gear oil in it and it hasn't leaked since.

Due to my experiences with it, when it comes to synthetic gear oil, I am of the opinion "if it ain't broke, don't fix it."


Tom Piper

Corvette Kid

I'm not sure about cars, though I would imagine there are some, but virtually all HD trucks have come with syn. lube in the transmississions and differentials for some years now. And for the same reason GM required it in our cars, it holds up to much more heat.

I would also think that any good quality sealer is made to be compatible with syn. lubes these days since they are in such wide use now.

And you are correct to thoroughly degrease the surface as a last step in surface prep. I usually use brake clean or a clean solvent, something that dries with no residue.

larryfs

my Denali has GM syn gear oil from the factory, and I am considering using it in my 96.

however my 03 Denali wasn't available with a posi rear.