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I’m getting mixed results with using gear oil that has the limited slip additive already mix in. I bought the synthetic 75-90 Lucas oil thinking this is some good stuff but it says it has the LS additive in it. I have the Z71 with the g80. I was wondering if this is safe to use or should I return it and get something else?
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I’m getting mixed results with using gear oil that has the limited slip additive already mix in. I bought the synthetic 75-90 Lucas oil thinking this is some good stuff but it says it has the LS additive in it. I have the Z71 with the g80. I was wondering if this is safe to use or should I return it and get something else?
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Replace it. I made the mistake and used one with an additive in it and it stopped the G-80 from locking up. I drained it and put the factory oil in and problem solved. Plus the factory oil is 100% full Synthetic.


 

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Replace it. I made the mistake and used one with an additive in it and it stopped the G-80 from locking up. I drained it and put the factory oil in and problem solved. Plus the factory oil is 100% full Synthetic.


The factory ATF (Dexron-VI) is synthetic, but the gear oil is not. The gear oil you linked is conventional, the synthetic gear oil from GM/ACDelco will say synthetic on the bottle. You can also go to https://public.spheracloud.net/Sear...aJ3qYLT7QA&companyid=1IgGao56XIQA&method=home, search for "75w-90", and in the results you'll see the synthetic gear oil is different than the conventional.

FWIW, Phillips 66 produces the conventional 75w-90 gear oil and Chevron makes the synthetic 75w-90 gear oil.
 

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The factory ATF (Dexron-VI) is synthetic, but the gear oil is not. The gear oil you linked is conventional, the synthetic gear oil from GM/ACDelco will say synthetic on the bottle. You can also go to https://public.spheracloud.net/Sear...aJ3qYLT7QA&companyid=1IgGao56XIQA&method=home, search for "75w-90", and in the results you'll see the synthetic gear oil is different than the conventional.

FWIW, Phillips 66 produces the conventional 75w-90 gear oil and Chevron makes the synthetic 75w-90 gear oil.
I believe you're incorrect. I know there was two different versions one full syn and one dino. This is the full synthetic version of it.
 

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I believe you're incorrect. I know there was two different versions one full syn and one dino. This is the full synthetic version of it.
Maybe, I just don't know why GM would specifically separate them, one with synthetic in the name and the other without, with different part numbers, if they're all synthetic. Seems more like a labeling mistake, like when Mobil listed the Dexron LV fluid as being backward compatible with Dex6 when it wasn't.
 
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Sorry for the crappy PIC I'm at work and I didn't take it, I found it on-line but I read it on the back of my container.
Based on these listings and their price, it looks like you're correct. GM is dumb with their product naming/labeling.

 
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Based on these listings and their price, it looks like you're correct. GM is dumb with their product naming/labeling.

Yup I totally agree. When I screw up and put the wrong stuff in and wanted to swap it out with the factory oil I went down the WTF rabbit hole on this one.
 

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I may be way off, but I thought that my Z71 had a locking diff. I also thought that a locking diff was a totally different beast than a limited slip diff.

I am obviously not intricate here, so could anyone anyone set me straight?
 

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I may be way off, but I thought that my Z71 had a locking diff. I also thought that a locking diff was a totally different beast than a limited slip diff.

I am obviously not intricate here, so could anyone anyone set me straight?
It should have the Eaton G80 locker. you can confirm by looking at the "RPO" codes and I would bet the G80 is listed.
They're are videos on youtube showing how the G80 functions.
 

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I may be way off, but I thought that my Z71 had a locking diff. I also thought that a locking diff was a totally different beast than a limited slip diff.

I am obviously not intricate here, so could anyone anyone set me straight?
The G80 is a mechanical, automatic locking differential. The locking mechanism functions based on 2 conditions; A left-to-right speed differential of ~120RPM and a road speed of less than 25MPH (IIRC. Might be 20MPH). If both of these conditions are met then there is a pawl that engages the mechanism that engages the clutch which quickly ramps up until the differential is fully locked.

It is not like a Detroit or other aftermarket automatic locking differential that functions based on torque input because those are too unpredictable and unsafe for daily-driven street vehicles. The clutch-based locking mechanism in the G80 makes it predictable, safe, and relatively smooth to engage. Using a gear oil with limited slip additive will prevent the clutches from allowing full lockup and it either won't lock up at all or will be inconsistent/unpredictable. GM has had a bulletin about this since shortly after the G80 was an option in trucks. A G80 in most/all GM cars isn't a locker but is a limited slip so having the same RPO code for a locker and LSD combined with the locker using clutches makes for some very confused people.
 
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The G80 is a mechanical, automatic locking differential. The locking mechanism functions based on 2 conditions; A left-to-right speed differential of ~120RPM and a road speed of less than 25MPH (IIRC. Might be 20MPH). If both of these conditions are met then there is a pawl that engages the mechanism that engages the clutch which quickly ramps up until the differential is fully locked.

It is not like a Detroit or other aftermarket automatic locking differential that functions based on torque input because those are too unpredictable and unsafe for daily-driven street vehicles. The clutch-based locking mechanism in the G80 makes it predictable, safe, and relatively smooth to engage. Using a gear oil with limited slip additive will prevent the clutches from allowing full lockup and it either won't lock up at all or will be inconsistent/unpredictable. GM has had a bulletin about this since shortly after the G80 was an option in trucks. A G80 in most/all GM cars isn't a locker but is a limited slip so having the same RPO code for a locker and LSD combined with the locker using clutches makes for some very confused people.
I had my idea about it's function directionally correct, but really appreciate your fill in. I read your comment twice to imbed. So, might D Max be running the wrong goo?
 

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I had my idea about it's function directionally correct, but really appreciate your fill in. I read your comment twice to imbed. So, might D Max be running the wrong goo?
Yeah, the Lucas oil is the wrong stuff since it has LS additive which is why a couple of us have recommended synthetics that will work since they have no additive.
 
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I’m getting mixed results with using gear oil that has the limited slip additive already mix in. I bought the synthetic 75-90 Lucas oil thinking this is some good stuff but it says it has the LS additive in it. I have the Z71 with the g80. I was wondering if this is safe to use or should I return it and get something else?
View attachment 9565
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Go with Red Line, you'll be glad you did.
 

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Go with Red Line, you'll be glad you did.
No, they won't. Red Line 75w90 contains friction modifiers for LSDs which will cause the G80 to not lock properly.

Contains additional friction modifiers for suitability with clutch-type limited slip differentials - for most LSDs, no additional friction modifiers are required
The only Red Line gear oil that doesn't contain modifiers is their 75w140NS gear oil which is not the correct viscosity. I get people have various brand loyalties, but that should not mean that the actual requirements are ignored. The G80 will not work correctly if there are friction modifiers in the gear oil. GM has an official statement saying the same thing.
 
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No, they won't. Red Line 75w90 contains friction modifiers for LSDs which will cause the G80 to not lock properly.



The only Red Line gear oil that doesn't contain modifiers is their 75w140NS gear oil which is not the correct viscosity. I get people have various brand loyalties, but that should not mean that the actual requirements are ignored. The G80 will not work correctly if there are friction modifiers in the gear oil. GM has an official statement saying the same thing.
OK, whatever. Ive used it in about 300 vehicles with Posi rear ends and locking rear ends; all work just fine, zero issues. Some of them actually had water in the rear end lube when it was drained... yucky stuff. Part of it is that I do use the 75W/140 in some cases as well. Specifically if the truck is being used for towing, or is always carrying a heavy load.
 

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OK, whatever. Ive used it in about 300 vehicles with Posi rear ends and locking rear ends; all work just fine, zero issues. Some of them actually had water in the rear end lube when it was drained... yucky stuff. Part of it is that I do use the 75W/140 in some cases as well. Specifically if the truck is being used for towing, or is always carrying a heavy load.
The G80 is unique in that it uses clutches for the locking mechanism. Where most LSDs need the friction modifiers to avoid clutch chatter and most lockers don't care one way or the other because they are either selectable or mechanical and have no clutches, the G80 is different and it's specifically the G80 (Eaton mLocker) that requires NO friction modifiers in order to work properly. The friction modifiers cause the gear oil to be too slippery and will prevent the G80 clutches from locking fully or properly. You can "whatever" all you want, but this is a fact and not only have owners complained about the G80 working erratically or not at all after servicing the diff with the wrong fluid but as I said GM has an official bulletin that states very clearly that absolutely NO friction modifiers are to be used with the G80 because of the reasons mentioned here. If you have a problem with this then take it up with GM.
 
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