I thought I'd pass along this info I found over at bobistheoilguy.com, because I found it very helpful. I about passed a brick when I first read the post, but it looks like all is well in my case.

Moral of the story: Mobil 1 75-90W is perfectly fine to use in a D-Max rear end. This is what I use.

Mobil SHC gear lubes or any other type of E500 gear lubes will eventually toast the clutch plates in your locker.

Hope this helps.

http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=16;t=001519

I read the link, but did not come to the same conclusion. I saw nothing in the post stating that Mobil 1 meets GM's spec. I did see that Royal Purple has a product that does meet GM's spec. I also noted that using the wrong lube can cause problems.

The mobil 1 does meet GM specs

I have 76k miles on Mobil1, changed at 1500 and no problems at all.

I just changed yesterday. I about passed a brick also uz I put Mobil 1 in there. I wonder why I never even thought about compatablity....anyway, time to double check the lable....

so what if i put the amsoil in there and there additive? am i doing bad or good?

so what if i put the amsoil in there and there additive? am i doing bad or good?Use the Amsoil without the additive. We do not need the additive as the are not clutch packs in our rears. The gov-lock is fifferent than the old posi-rears that used clutch packs.

I have have Amsoil since 2,000 miles and have over 40,000 miles without any issues. And my truck does more than just run up and down the road, it gets to really pull stuff also.

Mobil1 75W90 gear lube has a GL5 rating......................GM recommends a fluid that meets or exceeds the GL5 rating.

so what happens if i already have the additive in it?

The Petro-Canada Traxon E synthetic gear lube was designed for eaton manual trans and rear ends. This is what I use and it seems to be doing a good job.

This topic has been around since the 01s came out. GM's spec is had to match to the letter but there are a lot of people doing just fine with the brand name synthetics. Eaton mentions on their site that other brands meeting GL-5 lubes are acceptable. The clutches may chatter a little but it doesn't harm anything. Later! Frank

We do not need the additive as the are not clutch packs in our rears. The gov-lock is fifferent than the old posi-rears that used clutch packs.

You might want to double check your source, the G80 is a clutch type limited slip. It is very different in function from the Gov Loc GM used in earlier models but it still a clutch engaged limited slip.

The reason GM does not recomend a friction modifier is because the factory specified lube already contains the modifier.

If the lube you used meets all GM specs you are safe. If the lube you used does not meet GM specs you might have to add a friction modifer to keep the clutches quiet.

I changed mine to Mobil 1 at 2500 and have had no problems. I put 90k on my last Duramax and it too ran Mobil 1 from 2500 on with zero problems.

OD

You might want to double check your source, the G80 is a clutch type limited slip . . . .
OD
You need to do some checking. . . the G80 is not a limited slip. . . go read your owners manual . . . it is a locker . . . . do a "G80" search on the forum and you will find several threads about G80 lockers.

Also, if you take your diff cover off you can verify that there are no clutches in there :)

The G80 is a locker, and it does have clutch packs. It does not need a friction modifer since it is designed to be either locked or unlocked - it does not function as a limited slip. If you want to prove this to yourself, just jack up the rear end and rotate a tire. The opposite tire will spin backward just like an open carrier. The friction modifier is intended to reduce or eliminate clutch chatter in a clutch type limited slip where the clutch packs are preloaded at all times and need to slip when cornering. This is not the case with the Eaton automatic locker (G80 RPO) that is used in our trucks. People get confused when they go to the Eaton web site because they start reading the information that pertains to the clutch type limited slip units that Eaton has made for many years (and also used to be used by GM under the G80 RPO in the '60s and early '70s).

So whats the best lube, I'm getting ready to do mine. $22.00 for the GM lube dose not bother me, since I paid 40K for the truck and $50 per week in gas. Should I use mobil 1 or the GM grape juice ?

You better start putting diesel in this one instead of gas.):h

I just picked up 6 quarts for my truck and got them for $23.10 each. Granted, it's more than Mobil 1 from Schucks, but then again, at least GM can't deny a warranty claim based upon the use of the "wrong stuff". Like you said, it's a $40,000 truck...is $23.10 per quart going to kill anyone...granted, it's overpriced but then again, so was the truck ;)

The G80 is a locker, and it does have clutch packs. It does not need a friction modifer since it is designed to be either locked or unlocked - it does not function as a limited slip. If you want to prove this to yourself, just jack up the rear end and rotate a tire. The opposite tire will spin backward just like an open carrier. The friction modifier is intended to reduce or eliminate clutch chatter in a clutch type limited slip where the clutch packs are preloaded at all times and need to slip when cornering. This is not the case with the Eaton automatic locker (G80 RPO) that is used in our trucks. People get confused when they go to the Eaton web site because they start reading the information that pertains to the clutch type limited slip units that Eaton has made for many years (and also used to be used by GM under the G80 RPO in the '60s and early '70s).
The G80 is not a locker. We can argue all day what it is but it is not a locker. I spent many years R&R rearends and I have rebuilt many Eaton G80s as well as many GM GovLocs (earlier RPO G80s). The G80 is nothing more than an agressive limited slip that provides a marginal mechanical engagement when wheel speed equalizes under 20 mph. It looks nothing like a modern day "locker" and was never designed to function as such. It's designed to be a compromise between the performance of a locker with the on road characteristics of a Limited Slip and it accomplishes this very well.

OD

The G80 is considered a locker by the manufacturers, the following was cut and pasted from the Eaton website:

The Eaton Locker is option code G80 Locking Differential on GM light trucks and SUV's. Go to Chevrolet, GMC, Cadillac or Oldsmobile websites for more information.

A Quick Look At How It Works...

We'll start by pointing out that the Locker is a speed sensitive design. That is, it reacts to wheel slip by sensing when one wheel is spinning substantially faster than the other.

So, when you're cruising along on clean dry pavement, the locker operates like any regular open differential.

But, as soon as wheel slip happens, going forward or reverse, the locker immediately kicks in. Here's how.

The differential is set up with a flyweight governor that responds to differences in wheel speeds, and disc packs that are mounted between the side gear and the case.

Whenever one wheel is spinning substantially faster than the other, the governor spins rapidly, causing the flyweight to open. That flyweight then catches on a latching bracket and the lockup process begins.

During lockup, a self-energized clutch system causes a cam plate to ramp against a side gear. This ramping action compresses those disc packs mentioned earlier. The ramping continues until both axles - and therefore both wheels - are spinning at the same speed. This is full lock, and it prevents any further wheel slip. (Note: Axle lockup can only occur at speeds below 20 mph.)

The entire lockup process takes about a split second, and is virtually unnoticeable by the average driver. When both wheels regain traction, unlocking occurs and things go back to normal.

Everything else is just an opinion.

The G80 is considered a locker by the manufacturers, the following was cut and pasted from the Eaton website:

The Eaton Locker is option code G80 Locking Differential on GM light trucks and SUV's. Go to Chevrolet, GMC, Cadillac or Oldsmobile websites for more information.

A Quick Look At How It Works...

We'll start by pointing out that the Locker is a speed sensitive design. That is, it reacts to wheel slip by sensing when one wheel is spinning substantially faster than the other.

So, when you're cruising along on clean dry pavement, the locker operates like any regular open differential.

But, as soon as wheel slip happens, going forward or reverse, the locker immediately kicks in. Here's how.

The differential is set up with a flyweight governor that responds to differences in wheel speeds, and disc packs that are mounted between the side gear and the case.

Whenever one wheel is spinning substantially faster than the other, the governor spins rapidly, causing the flyweight to open. That flyweight then catches on a latching bracket and the lockup process begins.

During lockup, a self-energized clutch system causes a cam plate to ramp against a side gear. This ramping action compresses those disc packs mentioned earlier. The ramping continues until both axles - and therefore both wheels - are spinning at the same speed. This is full lock, and it prevents any further wheel slip. (Note: Axle lockup can only occur at speeds below 20 mph.)

The entire lockup process takes about a split second, and is virtually unnoticeable by the average driver. When both wheels regain traction, unlocking occurs and things go back to normal.

Everything else is just an opinion.
Marketing hype is improperly using the traditional term of "locker". The original locker was the detroit no-spin locker. It used no clutches and provided a 100% mechanical axle to axle engagement at all speeds. Under certain conditions a detroit will unlock to increase manuverability.

I guess you just have to see the two side by side and understand how they work. They are very different from each other and are definately not the same thing. The G80 being a hybrid does not fit 100% in to either category. The G80 still uses friction clutches to create the so called "lockup". The G80's function and construction more closely resemble a limited slip than a locker.

Marketing hype is improperly using the traditional term of "locker". The original locker was the detroit no-spin locker. It used no clutches and provided a 100% mechanical axle to axle engagement at all speeds. Under certain conditions a detroit will unlock to increase manuverability.

I guess you just have to see the two side by side and understand how they work. They are very different from each other and are definately not the same thing. The G80 being a hybrid does not fit 100% in to either category. The G80 still uses friction clutches to create the so called "lockup". The G80's function and construction more closely resemble a limited slip than a locker.

I agree to disagree, it's a hybrid locker, taking the best of both worlds. The more difference between axle speed the greater the pressure on the clutches. Wheel slip is what causes this differential to lock unlike a limited slip differential which uses input torque to apply pressure to the clutches. The fact that the G80 uses tire slip to activate the clutches makes it a locker not a limited slip. The only way to get a better lock is to use an elock differential or a spool.

I agree to disagree, it's a hybrid locker, taking the best of both worlds. The more difference between axle speed the greater the pressure on the clutches. Wheel slip is what causes this differential to lock unlike a limited slip differential which uses input torque to apply pressure to the clutches. The fact that the G80 uses tire slip to activate the clutches makes it a locker not a limited slip. The only way to get a better lock is to use an elock differential or a spool.
I think we have both killed the horse...................:D

But, the fatal flaw to the G80 is the clutches. I consider it a limited slip because friction clutches provide the locking mechanism. It is not a true mechanical linkage between axle shafts like a traditional locker or elocker. In addition, under severe loading the holding threshold of the clutches can be overcome and bye bye "locked" hello limited slip.

I think we disagree on what designates a locker. :)

So, is it safe to use mobil1 or should we stick to the grape juice.

So, is it safe to use mobil1 or should we stick to the grape juice.
):h

I run mobil 1 and have for many miles.........................

I like ):h a spirited conversation that does not resort to name calling... thank you! I have not changes my gear lube yet, but I will probably use grape juice..

leftmy8.1,

In my humble opinion, I believe differential oil is like investing...there are some who throw caution to the wind and buy aggressively and there are others who are more conservative in their approach - neither of them are bad, they just have different results and different comfort levels. Do what you feel is to your comfort level. For me, I went with the high dollar grape juice from GM as I don't want to take a chance with a $44,000 truck. For others, they use Mobil 1 which is a fine product and will probably perform the same as the GM grape juice but they run the risk should something happen and GM start talking warranty issues.

It's all about your comfort level...do what doesn't disturb your sleep at night :)

I have searched high and low but I cannot find what exactly IS this GM spec that no one by GM meets.

From the Eaton Web Site:

What type of lubricant and limited slip additive should I use with my newly purchased Eaton Limited Slip Differential?

Eaton strongly recommends utilizing an API-GL5 approved Mineral-Based 80W-90 Axle Lubricant and one 4-oz bottle of Limited Slip Additive (GM or Ford).

From the Eaton Web Site:

What type of lubricant and limited slip additive should I use with my newly purchased Eaton Limited Slip Differential?

Eaton strongly recommends utilizing an API-GL5 approved Mineral-Based 80W-90 Axle Lubricant and one 4-oz bottle of Limited Slip Additive (GM or Ford).
The G80 in the new trucks is not an "Eaton Limited Slip", it is an Eaton locker. The limited slip is another product that Eaton makes and has made for many years. You do not use limited slip additive with the locker. GM specifically specs a synthetic (not mineral based) lubricant in the rear differential of these trucks. I have Mobil 1 in mine.

From what I see at the eaton web site it looks like the clutch discs are used to contol the engament of the locking pins and not being used to handle the actual load at the wheels. I think that is whats confuzing everybody. Once the pins are engaged it is truly mechanically locked. Unlike a limited slip when the plates carry the load at the wheel.

http://www.traction.eaton.com/prod2.htm

The G80 in the new trucks is not an "Eaton Limited Slip", it is an Eaton locker. The limited slip is another product that Eaton makes and has made for many years. You do not use limited slip additive with the locker. GM specifically specs a synthetic (not mineral based) lubricant in the rear differential of these trucks. I have Mobil 1 in mine.

So don't use the additive. I just posted what the WEB site specifies. I am not trying to argue one way or the other. As I stated in a previous post - I will use GM grape juice. I often wonder if you people read the posts.

arguy, the page you refer to is a little deceiving. It starts off talking about the G80 and changes to limited slip where you got the oil spec. I saved this on my computer because I new this subject would be coming around again. This info came from Tex Doc on the diesel page before we had this forum. Hope it helps again. Later! Frank

And this,
"Back in November, I received the following:
"From Mr. Ralph Holmquist of Eaton, the maker of the locking differential:
"The maintenance schedule for the rear axle was developed by American Axle &
Manufacturing and GM truck based on multiple tests. The Eaton locker does
not require additional maintenance nor does it add heat to the lube. The
lube will darken due to the carbon wear on the clutch surfaces, much the
same as a disc brake pad & rotor. This does not damage axle components such
as seals or bearings. However, a new axle can produce excessive
temperatures (plus 350 degrees F) due to the ring & pinion breaking in that
will break the lube additives down. Avoid high loads, trailer towing and
high speed extended driving during the initial break in of the vehicle.
After the break in period axle temps will level at a much lower figure.
Lube changes are a good idea because the additives are replenished and
contaminates such as casting sand are eliminated. The axle is filled at the
factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276.
The GM service # is 12378261. This is the only lube we have done extensive
testing with to insure locker compatibility. The only negative to using one
of these other lubes is an increased potential for clutch chatter. This
really doesn't hurt anything and can be corrected by changing the lube. I
noticed in the latest GM owners manual the term "or equivalent" when
referring to the lube specification. Look for a GL5 rating on the bottle to
make sure the ring & pinion, seals & bearings are protected. Limited slip
additive is not needed."
The following information outlines the type of
rear axle lubrication utilized in GM Truck axles.
I. Fluid Type
II. The 1500, 2500 and 3500 GMT 800 Trucks utilize SAE 75W-90 Synthetic Axle Lubricant. The GM part number is 12378261 and the specification is 9986115.
¨ Note: the recommended lube for HD trailer towing is a 75W-140 Synthetic Gear lube, part number 12346140.
III. Inspection/Change Frequency
¨ The 1500 and 2500 GM Trucks recommend rear axle fluid level checks every 7500 miles. Fluid should be added as needed.
¨ Under heavy duty trailer towing conditions the lube should be changed after the first 500 miles of towing. This is due to the extreme heat generated on break-in of the hypoid gear set."

[ <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><st1:date Month="10" Day="26" Year="2002">10-26-2002</st1:date>: From TxDoc over on the dieselpage ]

And one last note: Please use synthetic gear oil.

Thanks Frank, I agree on the synthetic gear lube. I wish Eaton would provide better information on there WEB site. I apologize to everyone for misinterpreting their information. Just trying to help.

Schaeffer's 741 75w-90 is Eaton approved and is what I have in both my diffs. Only draw back is you have to buy it in minimum 5 gallon bucket.

Wow! This topic ranks right up there with solving world hunger.

You would be supprised how fast that bucket goes when you tell people that it meets all Eaton requirements and its only 65.00 to do front and rear.

I usually keep 10 gallons on hand....

Schaeffer's 741 75w-90 is Eaton approved and is what I have in both my diffs. Only draw back is you have to buy it in minimum 5 gallon bucket.

Does Schaeffer's say they meet the GM (Grape Juice) spec?

Bill, no, it does not say it directly. Here is the tech info on it 741 (http://schaefferoil.com/datapdf/740-741.pdf)

Thanks Eric... The Schaeffer's specs look very good! :)

so in driving in sand conditions up steep hills the g80 engaged before 20mph will stay engaged obove 20mph as long as engaged before 20mph ?????