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

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Nick1994

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I have a 1996 Chevy C1500 with 157k miles and the 5.7L Automatic with the limited slip rear end and 3.73 gears.

I doubt the differential fluid has ever been changed and I called the dealer to ask what gear oil to use, they said 75w90 with a limited slip additive, but when I called another dealer this guy, who seemed like he knew what he was talking about said not to use a limited slip additive on it since it is not a true posi (or something like that) that the dealers got in trouble for using that additive, that it caused premature wear. So I called another dealer and they said to use 75w90 with the additive.

What do I do? Do I use the additive or not?

Which brand of gear oil should I buy?
 
Have you tried contacting GM directly? I'm interesting in seeing their response.

And if you're not supposed to use the LS additive, does that also mean you shouldn't use a gear oil that already contains the additive, like Mobil 1 Synthetic Gear Lube LS?
 
Here's a question to ponder.. if they don't want you to put a limited slip additive, should you be using gear oils that already have the LSD additive in it?

I've always been told, it doesn't hurt to add it, but don't add more than the 4oz. If you don't need it, you're throwing your money away but it wouldn't cause any issues.
 
Exactly, most of the gear oils at O'Reilly's say for use in a limited slip, so confused, feel stupid.
 
I didn't know the C1500 had the LSD option.

As for the premature wear, I wouldn't worry about it. With 220k miles on my '98, the rear differential still had the QC guy's initials wrote on it. Besides, most gear oils already have the additive in them.

Mobil 1 has worked the best for me.
 
Originally Posted By: Nick1994
I have a 1996 Chevy C1500 with 157k miles and the 5.7L Automatic with the limited slip rear end and 3.73 gears.

I doubt the differential fluid has ever been changed and I called the dealer to ask what gear oil to use, they said 75w90 with a limited slip additive, but when I called another dealer this guy, who seemed like he knew what he was talking about said not to use a limited slip additive on it since it is not a true posi (or something like that) that the dealers got in trouble for using that additive, that it caused premature wear. So I called another dealer and they said to use 75w90 with the additive.

What do I do? Do I use the additive or not?

Which brand of gear oil should I buy?


GM does not use limited slip differentials in the trucks.

Look in the glovebox for the RPO sticker and see if you have the code G80.

A G80 differential is a locking differential and does not require any type of additive. The fluid is the same regardless. Simply use a 75W90 and I recommend Mobil 1. The use of limited slip additive will interfere with the G80 locking mechanism and cause it to chatter badly.


Are you going to remove the differential cover or use a pump to suck out the fluid?
 
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I just got off the phone with the idiots at GM. The first guy I spoke to had such an accent I couldn't understand him and hung up, and called again. The next guy was obviously Amwrican, but didn't know what gear oil was for, and didn't seem to understand what a differential was. He said he'd call me back after speaking to some team.
 
Originally Posted By: Nick1994
I just got off the phone with the idiots at GM. The first guy I spoke to had such an accent I couldn't understand him and hung up, and called again. The next guy was obviously Amwrican, but didn't know what gear oil was for, and didn't seem to understand what a differential was. He said he'd call me back after speaking to some team.


Did you check to see if you have the RPO code G80?

Go get a differential cover gasket at the auto parts store. An OEM differential gasket is preferable. Don't use RTV and that is a crummy way to do it. My 2002 Ford Explorer Sport Trac I had to make a gasket as Ford had no gasket for the diff.

Go get yourself 3 quarts of Mobil 1 5W30 and fill it up until the fluid runs out of the fill hole.
 
Originally Posted By: Nick1994
I just got off the phone with the idiots at GM. The first guy I spoke to had such an accent I couldn't understand him and hung up, and called again. The next guy was obviously Amwrican, but didn't know what gear oil was for, and didn't seem to understand what a differential was. He said he'd call me back after speaking to some team.


You might do better to send an email. It will be presumably forwarded along until someone who can answer the question receives it.
 
I would use any 80w90 or 75w90 synthetic perferably. I have never used an lsd additive and have had no issues. I have used dino 80w90 and synthetic 75w90 (valvoline) you dont need any lsd adds, just throw some lube in there and youll be fine
 
I have a 1993 C1500 with all wheel drive. The owners manual has a list of the specified fluids on page 7-25. I use today's versions of fluids that were specified twenty years ago. The results have been ok.

1993 model year lists locking differential, but no limited slip differential.
 
Thanks, but all my owners manual could tell me was how to check the fluid level, and to check the service manual, which I don't have since the truck was bought over 17 years ago, haha. I bought some Mobil 1 75W90 LS Synthetic for $11.49 a quart at O'Reilly's and a gasket, we'll see how she does tomorrow.
 
As you have been told look for the G80 code on the RPO sticker. If it has G80 you are going to be fine with any gear oil with LSD additive in it.
 
The limited slip friction modifier additive is for the type of limited slip differentials with clutches inside them. Chevy used some LS diffs with a different mechanism. These do not need the additive. The amount already in the gear oil is no problem. Extra might be a problem.

Buy any good GF-5 differential gear oil that says it is for all differentials including limited slip. If your year requires synthetic, use that. Change the gear oil and take a drive. Make slow figure-8 turns. If it chatters you know that it has clutches, and the clutches need more additive, little by little until the chatter goes away. If it doesn't chatter, you're good.
 
A good way to tell if you need it:

(1) Go ahead and use whatever 75W90 you want, (2) go out in the morning when it is cold/cool out, (3) cut the wheel hard and slowly back up.

If there is significant noise, vibration, or harshness - then add 4oz of LSD additive.
 
Stop. STOP! The misinformation here is getting out of control ...


First and foremost, what kind of diff does the vehicle have? I am the first to admit I don't memorize every vehicle option from every OEM, so I have no ability to tell you what is in the truck.

However ...

I can speak to part of this issue that is so often misunderstood. The G-80 is the RPO option code for the "locking" differential from GM. It applies to the axles, regarless of size or application. It is a code to designate the use of a traction control mechanism; it is NOT specific to one differential part number. Therefore, you'll see the G-80 listed in 1/2 ton, 3/4 ton, and 1 ton trucks. They may not use the same diff, but they will be using a similar traction device.

The "G-80" most certainly does use clutches in it, and it is not a "locker" in the sense that it has direct mechanical bond between axles, such as an "air locker" or "e locker". The G-80 diff uses a speed sensitive flyweight system and pawl to engage or disengage a clutch pack very similar to a "limited slip" differential. You can google Eatons site and find an old video presentation on it with some efforts. But as sure as the sun rises, there are clutches in there! And it acts VERY similar to a traditional LSD. In fact, the G-80 is really nothing but a glorified LSD, in that it is only active part-time when conditions are met (above a certain wheel-to-wheel rpm delta but below a predetermined vehicle speed).

Here is a direct quote from an engineer at Eaton; it is several years old now but still just as valid:
Automatic Locking Differential Lubricants. [Note from Michael Asmussen, Torque Control Products Division of Eaton Corp.] We recommend the following lubrications for our locking differentials:
1)Texaco 2276; Synthetic 75 W90; Gm Part # 9986115
2)Texaco 9622; Mineral based 80W90; GM Part # 9985290
3)Texaco 2080; Synthetic 75W140 (heavy duty applications); GM part # 9985991
Note - All of the above lubes are preblended with friction modifier. No additional modifiers are necessary or recommended. As far as other lubes are concerned, any standard GL 5 lube will work, but the units perform optimally with the three listed above.



You will note that he specifically mentions the inclusion of "friction modifier" in the Texaco fluids, which GM/Eaton was using at the time. He clearly states that any GL-5 fluid will work well, so you don't "need" a synthetic fluid; dino's will work just fine. He even lists the mineral based fluid as one of the approved GM part numbers. But the reason he specifically mentions FMs is because "Looky, looky... there be clutches in there, Ma!"

As for the topic of FM, Amsoil actually has a nice write up here:
http://www.amsoil.com/techservicesbullet...%20revision.pdf
As you can read for yourself, the use of GM spec 9986115 is simply the OTC version of a 75w-90 syn with FM already added. That is why they state to not add FM; it's already in there! Too much FM is just as bad (if not worse) than not enough FM. Either way, the clutch pack won't operate correctly.

Whether or not you use syns is up to you. They are not needed nor required by GM. Syns are "recommended" but not "required" - make note of that! If you replace the fluid, and you have a G-80 diff, you will need some portion of FM for the proper operation of the "locker" (which really isn't a locker, but a fancy part-time LSD). I will also note that there are many dino fluids that pass the SAE J2360 test, which is a civilian version of the old MIL standard. These J-2360 fluids are even "better" than a sole GL-5 designation, because they speak to performance, as well as blend characteristics. IOW - many J2360 fluids will also meet GL-5, but not all GL-5s will meet J2360.

But the Eaton diffs are pretty robust, and just about any GL-5 will suffice well more than enough.


'Nuff said.
 
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Originally Posted By: Nick1994
I just got off the phone with the idiots at GM. The first guy I spoke to had such an accent I couldn't understand him and hung up, and called again. The next guy was obviously Amwrican, but didn't know what gear oil was for, and didn't seem to understand what a differential was. He said he'd call me back after speaking to some team.


This is because they are just clueless customer service DOLTS, NOT the real tech peeps, or engineers (I had the same results in the past).

Sadly, they will not let you speak to the real savvy technical people, nor godforbid, an actual engineer.
frown.gif
 
GM does call the G80 an automatic locking differential.

If one simply goes by the owners manual it never mentions adding any type of friction modifier (LSD additive). My owners manual states that the rear differential simply requires 75W90 synthetic axle lubricant.

In the case of a Corvette (with a REAL limited slip differential), GM specifies Dexron LS gear oil. I gather Dexron LS gear oil already has the proper friction modifiers.

I looked at a Camaro, and with the ZL1, they also specify Dexron LS gear oil while the SS requires a 75W90 gear oil and a limited slip additive.

Simply reading the owners manual provides a wealth of information.
 
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