Fluid for 2013 GM 4x4
- Thread starter robshu145
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should be good to go unless you got the G80 locking diff then you'll need a limited slip additive.
Looks like it calls for 75w-90. Brand is up to you. Any major well known brand should be fine.
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does it have the limited slip friction modifier
Might have to look at the tag with the option codes to decode what it was built with. There are some websites that can generate the original window sticker from the VIN, I had to do that to find out whether my truck had a limited slip or not. It won't hurt anything if you use a fluid like Mobil 75W-90LS that already has the LS additives in it even if you don't have a LS.
dnewton3
Staff member
For that application, any GL-5 in 75-90 will suffice.
I presume it has the "G-80" "locker" rear; that was a very common rear diff back in the day. It will need some FM in the fluid. SuperTech Full Synthetic has the FM already added, as do many others.
I presume it has the "G-80" "locker" rear; that was a very common rear diff back in the day. It will need some FM in the fluid. SuperTech Full Synthetic has the FM already added, as do many others.
Looks like you don't have the G80 code that would indicate a limited slip or locking differential. GU6 looks like a standard 3.42 differential.
https://decoderpo.com/
https://decoderpo.com/
You could anyway
dnewton3
Staff member
Completely incorrect and that thread you linked is rife with inaccuracies and falsehoods.
The G80 in the trucks IS a clutch driven device. It is only called a "locker" because the cam-ramps which clamp the clutch packs do so with enough force to fully "lock" (no slippage) for torque bias.
Go to the Eaton website and look at their product page. You'll see the "M Locker". That is the unit which was used by GM.
There was a GM document WAY BACK in the day where a statement was made to "not all friction modifier" to the lube for the G80. This was because the GM fluids (available at the parts counter) already had sufficient FM mixed in them, so you should not add more FM yourself, as too much FM can affect the desired torque bias. The document didn't say "no FM should be used." There is a gross misinterpretation of that document form GM. Amsoil even put out their own document to clarify the situation.
Most certainly, the G80 in trucks needs a small amount of FM in the gear oil or nuance chatter and locking events will happen.
Caveat Emptor !
This is the source of confusion. G80 is an option code for a limited slip. GM has used different types of limited slip device through the years on different vehicles. Some have clutches, some do not.
Every GMT G80 eaton supplied gov-bomb (Gov Lock) I've ever seen had pawls that engage the spider gears based on a rotatey spinny spring loaded governor thingy. For these, friction modifier is irrelevant. Not needed, but harmless. No clutch material to speak of. That said, I haven't handled/owned very many.
GM TSB #91-4-109 might be a good starting point.
To the OP - 75w-90 is appropriate for your vehicle. Use a quality GL5 fluid you can find at a good price. Pay no attention to "limited slip additive" markings on the package. Valvoline, Supertech, Mobil, etc.
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dnewton3
Staff member
Again ... I disagree.
The M-Locker (G-80) in the trucks is absolutely, without any doubt whatsoever, a CLUTCH DRIVEN LIMITED SLIP device.
Watch this video. It clearly describes the clutches and also the locking pawl function.
NOTE: they call the clutches the "active disk pack", and they talk about "friction discs"
If you don't believe the video, then maybe this literature from Eaton will help out ...
https://www.eaton.com/content/dam/e...ol/mlocker/eaton-mlocker-brochure-2009-en.pdf
And I quote: (my emphasis in bold)
Principle of Operation
• Wheel speed difference (left to right) in excess of 100 RPM will cause a flyweight mechanism to open and engage a latching bracket
• The stopped flyweight will trigger a self-energizing clutch system, which results in the cam plate ramping against a side gear
• Ramping increases until both axles turn at the same speed (full lock), which prevents further wheel slip
The confusion for most people comes in that the "locking" term is in the action of the little pawl which engages the mechanical cam/ramp. The little pawl "locks" the triggering arm to start the cam-ramp. It's not a "locker" in the traditional sense ( like a "Detroit Locker" ). The locking action of the pawl does NOT affect torque bias directly; it is only the tool that initiates the clutch pack being tightened. The clutches are what provide the torque bias.
Another part of the confusion is that the G80 (M-locker) actually acts as a LSD at all times. There's always a small amount of force on the clutch pack. Hence, the clutches are always slipping in a turn. It's only when the wheel speed differential is great enough does the pawl activate, which in turn clamps down tighter on the clutch pack. It's not that the clutches are free-spinning at all times; rather, they provide some bias at all times. It just gets "more" torque transfer when the camp ramp presses into the clutch pack with more force. Because the clutches do see a frequent amount of difference relative to wheel speed, this can cause high heat and alter the fluids. The FM is needed to keep a reasonably constant friction coefficient throughout that process. Without FM present, the clutch pack can experience inadvertent events of grabbing/releasing ("chatter" ) or occasionally cause an axle to "lock" when it shouldn't; results in binding of the axle in a turn where plenty of traction is actually present.
So, those clutches need some amount of FM in the fluid so they won't be "too" grabby (nusaunce locking events and chatter). The GM spec for their "grape juice" includes FM already in it. You can purchase the spec here if you so choose:
https://standards.globalspec.com/std/1400775/9986115
Here is a great TSB on the topic from Amsoil
https://www.syntheticoils.us/Files/TSB/Drivetrain/TSB DT-2004-07-01 GM Steerable Rear Axle Lubricant Revision.pdf
"The reason owners manuals, and Motor Information Systems both print the statement, “Do not add friction Modifier” is that GM specification#9986115, part #12378557 (USA) or #88901362(Canada) have the limited slip friction characteristics already in the formula, (Reference 5) and the addition of friction modifiers are not needed."
The GM lube spec 9986115 (applicable to these G80 "M Locker"s from Eaton) has the appropriate amount of FM already in it. One should not "add" more into the fluid. But that does not mean FM isn't in there! This would be no different than talking about other formulas. One does not need to "add" more flour to a box of premade cake mix. One does not need to add more salt to a bottle of Zing-Zang Bloody Mary mix. One does not need to add more bourbon to a can of premixed Beam branded "Old Fashioned" alcoholic beverage.
I do agree it's not detrimental to the M-locker to not use FM; it won't destroy the differential if you don't use FM. But what happens is that fresh fluid which does not have some FM in it can create a condition where the clutch pack will nusaunse lock and/or chatter. Essentially the fluid doesn't provide the proper amount of slip without FM. It's like Goldilocks; too much or too little is bad. Only the proper amount is "just right". The only way to make the problem go away is to add some FM into the fluid (like GM did with spec 9986115) if and only if the fluid chosen does not have FM in it to begin with.
Back when I had my 2006 Duramax 3500, I actually had this experience personally. After a fluid change, I had some odd/occasional clutch chatter in the G80. Merely added a couple of ounces of the FM, and after a few turn cycles, all was fixed.
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