Mobil 1 Gear Oil Does Not Meet GM Spec

[k1rod]

I want to do the first flush out of the rear differential in my new Silverado. I have the Eaton G80 locker. GM requires that you use a gear oil that meets GM spec 9986115. Reading the forums, I see a lot of people using Mobil 1 and Amsoil but I don't see the GM spec listed on either product. Today I called the Mobil 1 product support line. The Mobil tech guy I spoke with said Mobil 1 Synthetic Gear Oil DOES NOT meet GM spec 9986115 and that Mobil does not have any product that does meet this spec. I like Amsoil products also and I would call their number and ask the same question but my impression is that they tend to state that they meet any particular spec by default. I suspect the Mobil 1 and the Amsoil products are OK in this application since so many people here seem to be using them sucessfully. There must be some difference however because something is driving Mobil to avoid signing up for this particular spec. I guess I will order the expensive nasty purple stuff because although the other products may work fine, I think your giving GM a big out on warranty repairs on the differential if you don't. Especially if the manufacturer of the lubricant you are using explicitly says "They do not meet the required spec"

[Colorado Kid]

The way you're leaning is the way I decided to go. Since the genuine GM stuff smells like grape kool-aid mixed with a rotten egg you won't fool anyone into thinking that you used the right stuff if you didn't. You only need to swap it once ever, so you might as well use the right stuff.

Obviously others have reached different conclusions, and I don't think they're doing any harm with their GL-4 rated synthetics, but who really knows?

[mdrag]

Tx Doc's post (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=7;t=002814) has some good info on rear diff lube. GL-5 rating should be OK.

[JimWilson]

I contacted Mobil about the same thing, over 2 years ago. They said that their ATF and gear oil all met GM specs, and were perfectly fine to use. I saved the email, just in case... :D

[Jim Brzozowski]

Amazing, Mobil 1 synthetic gear lube has been working for years in Roger Penske's race cars, even those 800 HP turbo ones when he was in CART, I don't ever remember seeing one of his cars fail because the gears locked up. I'd be curious to know what particular part of GM's Spec. Mobil 1 does not meet. Does anyone know?

[HESS101]

I think its smart to get the old stuff out ASAP. When I removed my fluid it had bits of sand casting in it and metal pariculites on the plug with only 5K miles.You'd think they would do a better job of cleaning after they come out of the molds, but that's what they have us for.
However, I replaced the grape juice with GM #12346140 which is fully synthetic 75W140 for heavy duty towing. List was $32.02 per qt. My dealer let me have it for $15.56/qt.

[k1rod]

Amazing, Mobil 1 synthetic gear lube has been working for years in Roger Penske's race cars, even those 800 HP turbo ones when he was in CART, I don't ever remember seeing one of his cars fail because the gears locked up. I'd be curious to know what particular part of GM's Spec. Mobil 1 does not meet. Does anyone know? I don't think it has anything to do with how good a lubricant or how strong a film strength Mobil 1 gear oil has. I think it has something to do with having the correct coefficient of friction to make the locker work properly without occasionally chattering.

[jbplock]

FWIW I've been running the M1 75w90 for 33k miles and never noticed any chattering or other noises in the rear diff (changed the factory fill at 5k).

smile.gif

[Jim Brzozowski]

k1Rod, if that is the only hang up about Mobil 1, I'm not worried. Actually My first change was to Royal Purple, but when I noticed is said it contained friction modifiers for better clutch lockup on postive traction rear ends, I drained it out and put in Mobil 1. GM and American who makes the rear end for GM both say its a gear type lock up and no clutches are used and no friction modifier is required for these rear ends. So why run any more friction than required. I'll take the slick stuff on the gear surface which will make it run a lot cooler. I think thats the key.

[DmaxMaverick]

The problem with "other" gear lubes is the compatibility with seals and sealers.


SoTxPollock
Mobil makes the same claims as the rest about friction modifiers. Their product info states no need for modifiers for limited slip, or "Posi" type rear ends in most cases. Amsoil, Pennzoil, LE, etc. state the same.

I've been using Amsoil Series 2000 75-90 for over 60K with excellent results. My seals have lasted (and still lasting) longer than some that have use the GM stuff. It's ridiculous what the dealer charges for the stuff you can't get anywhere else. I won't pay into this monopoly.

[motovet]

Originally posted by DmaxMaverick:
The problem with "other" gear lubes is the compatibility with seals and sealers.


SoTxPollock
Mobil makes the same claims as the rest about friction modifiers. Their product info states no need for modifiers for limited slip, or "Posi" type rear ends in most cases. Amsoil, Pennzoil, LE, etc. state the same.

I've been using Amsoil Series 2000 75-90 for over 60K with excellent results. My seals have lasted (and still lasting) longer than some that have use the GM stuff. It's ridiculous what the dealer charges for the stuff you can't get anywhere else. I won't pay into this monopoly. DITTO!