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My experience doesn't jive with that. Literally 2 days after I went from AC Delco to Mobil 1 LS it started having issues unlocking and after a few weeks I pulled it and went to Amsoil Severe Gear.

Hasn't had the issue since. Maybe the Mobil 1 I got was not to spec?
 

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Just found this after calling my buddy (Stealership Service Manager)

Per GM tech bulletin

LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).

The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.

VEHICLES/COMPONENTS INVOLVED: ------------------------- ---- Some light duty trucks equipped with locking rear axles, RPO G80.
SERVICE PARTS INFORMATION:
Part Number Description ----------- ------------------ 10950849 Lubricant, Rear Axle (1 litre)
Parts are currently available through CANSPO.
WARRANTY INFORMATION:
As specified in Light Duty Truck Maintenance Schedules, locking rear axle fluid drain and refill is required owner maintenance at the first engine oil change. Failure to drain and refill the rear axle as specified may contribute to a later axle chatter condition. Refer to the appropriate Light Duty Truck Maintenance Schedule or service manual, section OB, for further details on change intervals.
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.
© Copyright General Motors Corporation. All Rights Reserved.
According to this, 80w-90 with NO limited slip additive is what our trucks take, regardless of the manual's instructions.

Might explain how people grenade them so often. Wrong stuff = bad.
 

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The additive allows the clutches (in a limited slip diff) to "slip more/grip less" so the clutches won't/don't "chatter" in a turn. If anything, the limited slip additive reduces surface tension/friction and has no effect on a locking diff or skeeters.
Stolen from somewhere else:

Before you can understand how friction modifiers function, it is important to understand how "wet" clutches perform.

In a wet clutch arrangement, there are three stages of engagement. During the first stage, the clutch is not in contact with the pressure plate or other metal plate. We will use an automatic transmission torque converter as an example. Anyone who has driven auto trans cars with lockup torque converters for a while has probably experienced a phenomenon known as "lockup shudder" or "torque converter shudder". Shudder is caused during torque converter clutch lockup by burnt fluid or fluid wich has exhausted all of it's friction modifers. The result is a chattering feeling when the torque converter goes into lockup mode. I will now attempt to explain the physics of wet clutch engagement.

As I have already mentioned, during the first stage the clutch is not in contact with it's mating surface. The fluid itself, however, is acting as a viscous coupling, causing a partial engagement. A side effect of this is heat, and I believe you all know that heat is the killer of automotive oils.

The second stage is very simlar to the first. At this point the clutch is very close, possible within thousandths of an inch, from it's mating surface. The viscous coupling is now more effective, but the pressure and shear load on the fluid are also higher, and the result is increased heat.

During the third stage, the clutch actually contacts it's mating surface and positive engagement is reached. The shear load of the fluid has been overcome and has either extruded itself outside of the clutch material or, depending on the application, has partly or entirely extruded itself through a porous friction material, thus exiting the engagement area of the clutch.

Now that we have an understanging of wet clutch engagement, lets see how that plays out in the real world. If a fluid has lost a substantial amount of the friction modifier, the shear of the fluid will be inconsistant accross the engagement surface and the clutch will briefly alternate between full engagement (stage 3) and viscous engagement (stage 2). The as power through the assembly varies, wich is connected in our case to a vehicle that we are inside of, a bucking of sorts is perceptable to it's occupants as power transmitted to the wheels is momentarely interrupted and regained.

In an application like a limited slip rear end, similar phenomena occur but in a slightly different manner. Because the clutch plates are constantly loaded with heavy springs, in theory they should always remain in stage 3 of lockup. If that were the case however, they would never slip. So the purpose of a friction modifier in a rear end is to ensure that the transition from S3 to S2 and back again during cornering etc. is smoothe. Therefor chatter occurs in much the same way it does in our torque converter clutch scenerio.

So the answer to your question if friction modifiers enhance friction or reduce it? The answer is there is no answer. Depending on what the application calls for and how it is engineered, they can do either.

You're welcome.
:D
 

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Go with the TSB.

No additive in a G80.

End of story.
No, not end of story.

We must discuss the fill plug on the G80 now.
:D
 

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When oils lose friction modifiers, the shear of the entire fluid is affected -- not just across the the engagement surface; there is no 'differential shear', the entire shear rate changes. I don't know enough about the G80 to add more to this conversation, but there are some flaws in the above argument concerning viscous coupling & shear rates concerning friction modifiers (part of my job is to model drilling fluids in the oil & gas industry).
 

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The inside wheel clutches in a turn need to slip smoothly or else there will be chatter or jerky differentiation in a turn. The addative allows the inboard clutches to slip smoothly. Addative is pnot needed unless limited slip chatter can felt/heard.
 

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We don't have a limited slip differential.

We have a full locking differential.

The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.
 

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If the out come is a trashed diff. Will make you a hell of a deal on a Tru Trac with install kit. No clutches to ever worry about again. :)
 

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Discussion Starter #31
cleaned the diff good and fresh gear oil solved the problem.
 

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So here I am in this what seems to be a big debackle over axle lube. Yesterday I decided to drain and fill my axles and after draining and filling the front I noticed that the Mobile 1 75w90 LS fluid I bought has the friction modifier included. I'm now wondering is it ok for the front axle and maybe not so much for the rear g80? I think I'm going to hold off on draining the rear til I get to the bottom of this. If it's ok for the front it will save me the hassle of draining and filling again plus the $24 in fluid I would waste. I'm sure I will get both answers here but thought I'd ask. Thx
 

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In the G80 use any full synthetic that has no modifier added . If you use anything else the differential will chatter and rear wheels will hop during turns . I would not use it in the front differential either .
 

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Yeah thought I'd get that answer. What synthetic have you found that has no modifier added?
 

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Amsoil 75W90 or GM 89021677 , updated part number 88900401 , are fine to use . Try driving your truck for a bit and see how the front diff reacts to the lube you put in . If it seems to be fine just drive it that way and save yourself some money and don't replace it until the next service interval. Almost forgot , Mobil 1 Delvac 75W90 is another choice that has no modifier added .
 

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Would like to get amsoil but my wallets kinda tight atm. Delvac not too available near me either. I did come across Chevron Delo 75w90 which is synthetic and not described as having fm added. I'm surprised that gear oil variety is so hard to come by. What's going on these days?

As for the front I'm just going to drain and fill it with Delo.

While we're on the subject of gear oil how does Pennzoil synchromesh work with our transfer case? I did that too. Hope I didn't go wrong with that fluid either.
 

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Yeah , a lot of product out there . I try to stay with what the owners manual states then I can't be too wrong . GM transfer case # 12345349 as per owners manual . Delvac you can get from Summit Racing at a decent price . I hear it's good stuff .
 

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Ok so every synthetic 75w90 on my 2 local auto parts store shelfs has friction modifier in it. I'm going to go with conventional 75w90 that has no friction modifier and easily available from the store and just change more frequently. As for the front I'm going to leave the Mobil 1 LS in since I don't see any harm in a mechanical engaging axle having it.

I did want to note that I've had the rear have a hard time disengaging a time or 2 last winter. I did have axle seals done at the dealer around the 30k mile mark 3 years ago which I'm wondering if they used incorrect fluid. I'm now at 46k miles.
 
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