Gear Oil- GM Limited Slip what to do???

[dnewton3]

I realize what GM calls the G-80, but that does not make it a real "locker" they way I was raised to think of them to functionally be. The G-80 is a glorified, part-time LSD, period. It most certainly has clutches, and it will not function well without FM. Actually, they don't make it; they buy it from Eaton, and then AAM puts it in for GM, but that's semantics ...

here is a youtube video of the G-80
http://www.youtube.com/watch?v=tTGZOJQQBeE
you can see in the video the clutch packs on both sides, and they speak of the ramping action against the clutches. And I quote " ... the active friction discs" .....
They speak of the friction disc packs several times. This is not a true "locker" like an Air or E-locker, folks. This is an LSD that uses a governed application mechanism to operate part time. Unlike a traditional LSD where the clutch pack is always engaged, the G-80 only actuates full force to the "friction discs" when the axial loads ramp out to put pressure on the disc packs on each side. When not activated, the ramping is relaxed and the clutches are only partially in play. That is why FM is needed; if it were not there, the clutches would grab and create chatter and/or nuisance locking, which is a common complain when folks do not use a fluid with FM in these units. Make no mistake, there are clutches in there, and they need FM.




As for the fluid, I would like to see the specific wording; I doubt it "requires" synthetics. I'm sure it "recommends" such, but rarely do they require it.

Most often, they state something like "recommend the use of a fluid that meets GM spec 9986115, part number ...." That "spec" is merely for a pre-mixed 75w-90 syn using FM, and the part number is the call out for the OTC convenient quart. Like the Amsoil link states, GM actually uses different specs for the factory fill, because they purchase separate lube and FM, and then mix it themselves at the factory. Whereas the OTC bottle needed to have a separate spec so that the blend knows exactly what to put into the bottle. In my 2006 GM truck manual, nowhere does it "require" the use of synthetic; it is only "recommended".

Ford manuals are similar; they push their products. But when it comes to engine oil, my Mercury manual specifically states that the use of synthetics is "not required". But that does not stop them from "recommending" their own Motorcraft synthetic by name.

 

[stchman]

Both GM and Eaton refer to the differential and an automatic locking rear differential.

As far as the fluid to put in, GM in the owners manual specifies a synthetic 75W-90 rear axle lubricant.

Taken from the 2013 Silverado owners manual:
Quote:

Rear Axle - SAE 75W-90 Synthetic Axle Lubricant (GM Part No. 89021677, in Canada 89021678).


Ever since GM went to the 5 year 100K mile warranty they have been specifying synthetic lubricant in the rear axles.

I have always used Mobil 1 75W-90 in the G80s I have owned and from the Mobil 1 website they say it contains friction modifiers for limited slip differentials.

 

[Whitewolf]

If you go to gmdexron.com you will find a list of 75W 90 both with and without fm. The fm fluids do not need additional fm, in fact it's a bad thing to do. If you can't find it PM me.

 

[dnewton3]

Yes, stchman, I realize what they call the G-80, but that does not really make it truly so. It is no more a true locking differential than my AR-15 is an "automatic" rifle. People are often careless with words, or they purposely use a wide interpretation of the word, for benefits they seek.

What I would agree with is that the "automatic locking" feature imples that the transmission of greater torque occurs when the unit reaches a point where activation of the flyweight system causes full engagement of the clutch pack. The flyweight "locks" onto the pawl, which causes the rod to turn the cam and ramp, which "locks" the clutches together fully, rather than partially ... However, the clutches will still have a point of torque transmission that can overcome them, even when fully "locked". The clutches create a higher capacity for torque balance value when fully engaged, but they most certainly are not "locked" IMO, because enough resistance to the clutch pack can still cause slippage, albiet a higher amount when fully ramped.

In contrast, look at a true "locker" like ARB or E-Locker. That, my dear sir, is a true locker, where no amount of force short of breakage of a component will thwart the two axles being paired together.

As for the fluid, the use of the term syntheics is symantics as well. I would rather have a fluid I know to be a strong performer, than a fluid that has a title on a bottle. Allow me to give you a few examples. Look at the PRI approved list of SAE J-2360 fluids; there are many dino GL-5s on there. Know what's not on there? Mobil 1 75w-90 is not on the approved list. Ironically, Mobilube 80w-90 dino oil is on the approved list. And it also has FM to boot. So you can use "syntethic" Mobil 1 75w-90 and presume it to be a decent fluid, or you can use their dino 80w-90 and KNOW it's tested and approved to that high performance standard. Ashland/Valvoline, OTOH, has both synthetic and dino products on the approved list, but not all their syns are approved (75w-90 Synpower is but 75w-140 Synpower is not). Mobil does have a syn on the list, but it's the Delvac syn diff fluid, not the Mobil 1 fluid. So in short, Mobil 1 75w-90 is not approved to J-2360, where many dino fluids are. You can see current listings here: http://www.pri-network.org/other-programs/automotive-qpl/lubricant/
And so before you go preaching about how great all syns are, you should make sure you research the full facts. I am NOT saying that Mobil1 75w-90 is bad; in fact I've used it many times myself with great satisfaction. But that product is GL-5 rated only, whereas others (including many dino fluids) are spec'd to J-2360 (a higher standard based upon performance).

Additionally, the actual statement in the 2013 GM truck manual states this:
On page 10-32 of the 2013 maual it states this:
"What to Use
Refer to Recommended Fluids and Lubricants on page 11-12 to determine what kind of lubricant to use."
And on pate 11-1 it states this:
"Maintenance intervals, checks, inspections, recommended fluids, and lubricants are mportant to keep the vehicle in good working condition."
Note the words "recommended fluids" (underlined by me); they are not "required".
So twice they recommend stuff, but never "require" it. In contrast, when it comes to engine oil, they require "licensed" dexos1 fluids. If a syn were so important for the diffs, they would call it out as "required" or "licensed"; they do not.



Does not really matter; we may never agree on this. Here's what's important ...
If the OP has a G-80, he should use a fluid with FM, because there be clutches in there! And if it's a G-80, any GL-5 fluid will work just fine, regardless of what GM's manual states, because the folks that designed and make the G-80 for GM (which is Eaton), have proven and publically stated it as such, and syns are NOT required for the safe, long lifecycle of the G-80. And his vehicle is way past any warranty concerns anyway.

 

[NHHEMI]

Originally Posted By: dnewton3
Stop. STOP! The misinformation here is getting out of control ...




The OP has a 1/2 ton and we are talking about the G80 used in the 1/2 tons. No misinformation here at all. You are overly complicating the thread now with your talk of G80's used in other applications and arguing wording semantics. The OP stated already he has a 1/2 ton. You could make the OP worry needlessly with all these short stories you have posted. The G80 used in GM 1/2 ton trucks, which is what we are talking about, is not fluid picky. Using a syn with LSD additive already in it does not hurt a darn thing in them.

 

[Jim Allen]

RPO G80 is a long standing GM Regular Production Option code that has encompassed several types of traction aids over the years (I think it first appeared in the '70s), from clutch type limited slips, gear type limited slips and the "locker" in question.

Since the '80s, G80 has most often been the "Eaton Automatic Locking Differential" in trucks, often known as the "Gov-Lock." It works just like Dave Newton describes above. It's basically a clutch type limited slip with variable bias and in the right circumstance, the clutches can be clamped tightly enough to provide something close to "lockup"... when the unit is new. As the clutch plates wear, they get farther from being able to do that. It isn't a true locker in the sense that it can achieve and hold 100% lockup, but it can be fairly effective in the right circumstances. It has many failings... but I won't digress...

As to lubricants, several of the OEM GM manuals I have from the '80s and '90s do require a special FM-treated lubricant. The GM Part Number is 1052271 or 1052272. These are old designations but they are spec'ed for many cars or trucks in those eras with a Posi or the Eaton or the G80 option. They don't specify an additional additive but the context is that this GM spec oil (now obsolete) contains a measured amount of FM. Non G80 axles spec plain GL-5 80W90 in the same books.

The Gov-Locks use in half-tons and higher are the same design and work the same way, I have see and repaired both, so anything that works for one works for the other. In the '90s, I think the Gov-Lock was the only traction aid offered in light rucks... regardless of truck capacity.

 

[wtd]

I have two 1998 chevy trucks. One is a 1/2 ton I bought new and the other is a 1 ton I bought in 2007.

Both have the G80 option. Neither one requires any FM to be added to the fluid according to the owner's manual. Neither one requires a synthetic.

I did use Mobil 1 diff fluid in the 1/2 ton for a while but after I had to have the pinion and carrier bearings replaced at about 75,000 miles, I went back to a conventional oil.

I doubt the OP's truck requires any FM. I would just put in the required viscosity and use a conventional oil and call it good.

Wayne

 

[Jim Allen]

Originally Posted By: wtd
I have two 1998 chevy trucks. One is a 1/2 ton I bought new and the other is a 1 ton I bought in 2007.

Both have the G80 option. Neither one requires any FM to be added to the fluid according to the owner's manual. Neither one requires a synthetic.

I did use Mobil 1 diff fluid in the 1/2 ton for a while but after I had to have the pinion and carrier bearings replaced at about 75,000 miles, I went back to a conventional oil.

I doubt the OP's truck requires any FM. I would just put in the required viscosity and use a conventional oil and call it good.

Wayne


Go back and look at your manuals more closely to double check. The service manuals I have here state that for the "locking" differential, RPO G80, does not require an FM added but it does require a special oil (GM #1052271 or #1052272) versus open diffs. This is an old oil spec but is an FM loaded oil.

Here is an exploded view of the G80 Gov-Lock. You can plainly see it has clutches. It stands to reason that it might need a FM modified oil but if GM doesn't mention adding an FM, it stands to reason theit oil recipe either has something special or has a very small amount of whatever they deem important. Something I vaguely remember, and the interested parties can research this, is that the latest Gov-Locks use carbon fiber clutches and that could very easily change the FM equation.

The one I had chattered on standard oil... when it worked, if it worked. It was pretty useless on the trail because (IIRC) it required you to have a 200 rpm difference in wheelspeed so that made it slow to hook up. Not so bad in mud. Terrible in rocks . The newerones needed only aout 100 rpm difference

 

[stchman]

Originally Posted By: wtd
I have two 1998 chevy trucks. One is a 1/2 ton I bought new and the other is a 1 ton I bought in 2007.

Both have the G80 option. Neither one requires any FM to be added to the fluid according to the owner's manual. Neither one requires a synthetic.

I did use Mobil 1 diff fluid in the 1/2 ton for a while but after I had to have the pinion and carrier bearings replaced at about 75,000 miles, I went back to a conventional oil.

I doubt the OP's truck requires any FM. I would just put in the required viscosity and use a conventional oil and call it good.

Wayne


My 2008 Silverado clearly stated in the owners manual to use 75W90 SYNTHETIC axle lubricant.

 

[Jim Allen]

Originally Posted By: stchman


My 2008 Silverado clearly stated in the owners manual to use 75W90 SYNTHETIC axle lubricant.


That stands to reason since many synthetic oils have improved friction characteristics over non-synthetic. That may be all that is required required for limited slip clutches. With some syns, you don't need to use any additional FMs, or much less of it at least.

Take note that some limited slip manufacturers don't want you to use synthetics because it can adversely effect clutch operation

 

[Whitewolf]

Let me see if I can help.

GM internal part#9986285 is for open diffs and lockers
GM internal part#9986290 is for plate type LSDds

They both have another number that can be obtained from GM CCA.

Be careful with the use of the term 'Synthetic' legally a Group III mineral base can be described as 'Synthetic' in North America so it is demoted to nothing more than a marketing term. In order to make any sense of it all you have to know the formulation and that information is not freely available.
Also bear in mind that sometimes the Manuals take time to catch up with latest fluid recommendations from Engineering.

 

[wtd]

Ok, I went back out and looked at the 1998 chevy truck owner's manual. Here is what is says. "differential front and rear axle lubricant(GM part no. 1052271) or SAE 80W-90 GL-5 gear lubricant" I also looked in my factory Helm service manual which does have the differential's separated by regular and locking and it says the exact same thing that I wrote up above under the section for locking differential. It makes no mention of a separate Friction Modifier needed. Both axles regular and locking take the same fluid with no additional FM.

The Dana rear axle in the HD3500 trucks require a 75W-140 synthetic rear axle fluid GM part No. 12346140 or equivalent and there again there is no mention of any additional FM.

Since the OP's 96 chevy truck with the locking diff is the same one that is used in my 98 with the locking diff, the only fluid he needs to use is an 80W-90 GL 5 rated gear lube. He can use a synthetic if he wants but it's not needed.

the local driveline shop that replaced the pinion and carrier bearings in my truck actually recommended to me to not run a synthetic in my year and type of diff because he had seen it cause some issues. Since I was running Mobil 1 synthetic axle lube in mine prior to my problems, I kind of believe him.

I hope this clears up the OP's question.

Wayne

 

[Jim Allen]

Originally Posted By: Whitewolf
Let me see if I can help.

GM internal part#9986285 is for open diffs and lockers
GM internal part#9986290 is for plate type LSDds

They both have another number that can be obtained from GM CCA.

Be careful with the use of the term 'Synthetic' legally a Group III mineral base can be described as 'Synthetic' in North America so it is demoted to nothing more than a marketing term. In order to make any sense of it all you have to know the formulation and that information is not freely available.
Also bear in mind that sometimes the Manuals take time to catch up with latest fluid recommendations from Engineering.


Thanks, Whitewolf. The only question is whether GM regards the Eaton "Gov-Lock a "plate style LSD" or not. It does have clutch plates so presumably.....

 

[Whitewolf]

Good question. I'll find out for you.

 

[dailydriver]

Originally Posted By: Jim Allen
Thanks, Whitewolf. The only question is whether GM regards the Eaton "Gov-Lock a "plate style LSD" or not. It does have clutch plates so presumably.....


As per your previous post asking about the clutch pack plates being carbon fiber or not, I believe that in EVERY current Eaton product which actually uses clutch plates, they ARE carbon fiber.

My Eaton (non locker) Posi most definitely has carbon fiber clutch plates.