range96

The reason I answered in this thread was to prevent you spreading false information. You do NOT need to add friction modifier to gear oil that does not contain it for the '19 ZR1. I stand behind my posts 100% and I'm done here!

Hib Halverson

The "LS" in Dexron LS means "limited slip". GM recommends that product for the ZR1 because the same lube can be used in just about any rear drive passenger car. Because it's a synthetic blend, it is less expensive than a full-synthetic but has better thermal stability than a petroleum-based gear lubricant. It's interesting to note that the only application GM recommends a full-synthetic rear axle lubricant is full-size trucks.

As for the friction modifier issue...to me it's not confusing. The ZR1 has an external eLSD so the friction modifier is irrelevant. GM knows that so it specifies Dexron LS because it doesn't need to catalog a special rear axle lubricant.

It is an acceptable gear lubricant for normal street use in any rear-drive light-duty passenger car. Since it has FM blended in, it works with any differential, open, mechanical limited slip or external eLSD.

Would I use it in my Corvettes?
Not a chance.

Another rear drive axle related issue worth discussing...

In the last several years, I have changed the lube in drive axles of a number of GM vehicles: 2016 ATS-V, 2018 Chevy Colorado and 2019 ZR1. In all those cases, I changed the lubricant at about 1000 miles and was surprised at how dirty and black it was. A friend of mine runs the Service Department at the Chevrolet dealer where I purchased my ZR1 and he says that he has noticed the same trend and recommends to all his customers with rear drive vehicles that they change the axle lube at about 1000 miles. Our assumption is that for unknown reasons, GM drive axles have a high rate of break-in wear.

Kracka

Here's the correct OEM rear differential gear oil:
It's a synthetic fluid made by Petro Canada.
product description:
Introduction
Petro-Canada’s DEXRON® gear oils are
synthetic extreme pressure (EP) automotive
axle lubricants formulated to provide excellent
long-lasting wear protection to extend
equipment life and reduce downtime and
maintenance costs. These API GL-5 quality
products are offered in the SAE 75W-90
viscosity grade and provide excellent year-
round performance. DEXRON® Gear Oil 75W-
90 is particularly suited for applications
requiring a synthetic gear lubricant that meets
General Motors 9986285 (or GMW16433).
DEXRON® LS Gear Oil 75W-90, with added
friction modifiers to perform in limited-slip
differentials†
, is designed for applications
requiring a product that meets General Motors
9986290 (or GMW 16445).
DEXRON® hypoid gear oils start with the HT
purity process to produce a 99.9% pure,
crystal clear base oil. By removing the
impurities that can hinder the performance of
competitive conventional oils, and blending in
specialty additives, DEXRON® gear oils deliver
maximum performance.
Features and Benefits
Overall Excellent Durability and Wear
Protection
• As a result of its anti-wear EP additives,
DEXRON® gear oils provide excellent
protection as proven by their performance
in the stressed L-37 and L-42 durability
tests. They can protect equipment being
driven longer, harder and faster in tougher
conditions for extended equipment life and
reduced maintenance costs.
The L-37 (ASTM D6121) durability test is an
axle dynamometer test that assesses fluid
performance under low speed and high torque
conditions. The stressed or high temperature
L-37 is run at 163°C/325°F for 16 hours
where parts are rated at the end of test for
wear, ridging, rippling, pitting/spalling and
scoring.
• DEXRON® gear oils provide superior
protection as proven by meeting or
exceeding the specification requirements
for all five parameters of the stressed L-37
test.

range96

^^^
The bottle does not say synthetic. The AC Delco 10-4016 88900401 says synthetic (no FM additive) on the label. The 10-4034 bottle doesn't say synthetic because it is really not a full synthetic oil.

People, let's not get wrapped up in the marketing speak. It is OK to use the recommended GM fluid in the '19 ZR1, but there might be better oils out there that can be used without the friction modifier.

HorsePowerObsessed

No one said it wasn't ok to use different fluids. The original question was about the exact type of AC Delco or OEM fluid he should use. He didn't ask anything about aftermarket fluids, he specifically asked for the the part number for the stock stuff.

Tamulinas2

Thanks Horse Power Obsessed: After carefully reading my owners manual, the correct Oil is GM# 88862624. I watched your video and my experience was
exactly like yours. The factory fill of the differential was about 1/2 quart low. The fluid was dark grey with silver swirls through it, My drain plug magnet had about
!/4 inch of metal particles on it. I'm really glad I got that stuff out of my differential. The transmission fluid looked OK and there wasn't any metal particles attached
to the drain plug magnet. But after a short drive, the shifting felt much smother. I'm heading to The Lone Star Le Mans tomorrow so I'll have a better idea about
the performance of the gear box.
I'm sending the differential and transmission samples to Blackstone for analysis. Maybe they can determine what the Grey Slim in the differential was. The GM
part looked nothing like it. I'll give an update when I receive the Blackstone Report. My current odometer is 5064,

HorsePowerObsessed

Awesome. I'd be interested in seeing the results from Blackstone. I plan on changing the differential fluid again come spring as well as the manual transmission fluid just to keep things running smoothly. Probably overkill, but it's cheap insurance so why not?

range96

Great! I agree.

You quoted my reply to Kracka... He claimed the GM oil is synthetic but the bottle does not says so. Other synthetic GM oils clearly state it.

RichieRichZ06

88861800 is manual transmission and transfer case fluid. Totally separate fluids and never interchangeable.

range96

RichieRichZ06 is spot on once again!

Kracka

I didn't claim it, the manufacturer does. All I did was copy/paste their data sheet. It's a quality product. Over-priced, but quality none the less.

TheRobSJ

Finally got around to doing this change. Wow that fluid was super contaminated. 3000 miles on the clock. Took exactly 3 quarts of Dexron LS.

Tamulinas2

Hi Horsepower: The results of the Blackstone Oil Analysis were a little surprising in that the main wear metals (iron, copper & aluminum)
were not what I would have expected from a break-in differential oil. Aluminum 66 ppm (11 ppm), Iron 85 (255), Copper 2 (7 ppm).
The values in parenthesis are Universal Averages from all GM differentials which show typical wear at 29,000 miles on the oil.
The iron is from gear wear. Copper is usually from bearing wear. Not sure the source of the aluminum.
The appearance of the oil was noted as being “odd” but viscosity was right where it should be. The technician could not comment on
the additive package of the oil but will be able to better compare when the Dexron LS Gear Oil (#88862624) in the differential now is sent in.
The transmission oil was unremarkable but the technician noted their Universal Average for this Corvette Transmission is extremely small.
I’d be happy to share the complete reports for my Engine Oil, Differential and Transmission at 5,059 miles. Just PM me.

HorsePowerObsessed

I would like to see it! I'll send you a PM.

MatthewMiller

I am reading this thread with interest. I see that Hib Halverson and range96 are saying that the diff fluid never touches the clutch plates - that the plates bathe in the ATF that presumably also runs the hydraulics for the active clutch locking. I'm pretty sure that's not accurate. Referencing the cutaway of the diff below, it looks to me like the clutches themselves are in the same continuous "chamber" of diff/gear fluid as the ring/pinion/spider gears. I'm pretty sure the the only thing the ATF is used for is to drive the hydraulics that apply locking force to the clutches. I believe the clutch packs bathe in the diff fluid, and that LS-type fluid or additive is absolutely required. To believe otherwise is to believe that:

1. The "bulkhead" between the ring gear and clutch pack is completely solid and contains a seal around the axle shaft that passes through it, which really makes no sense. That would be a crazy place to put a seal, where it's almost completely inaccessible.
2. The wet clutches bathe in ATF fluid that is also used by the hydraulic eLSD activation system. There is only 5.4oz of this fluid in the car, and that small amount would reside in the pump, hoses, and pistons of the actuation system. So the clutches can't really be wet in this scenario. Also this eLSD hydraulic fluid it is never changed for the life of the car (per the maintenance schedule in the manual). That really makes no sense!

That said, I'm open to some form of documentation or pictures/vids that prove that the clutches themselves don't touch the diff fluid. I've certainly been wrong before!

range96

Not, based on your picture.

MatthewMiller

What you see in the pic is one big chamber for all the mechanical parts of the diff, including the disks; and a separate hydraulic system comprised of a pump, the pistons that push the clutches together, and supply/return hoses to the pump. That hydro system has to be a closed system that contains high pressure for those pistons to do their work. Let's say that at peak clamping force there is 200psi of hydraulic pressure in that system. There's no way it could also supply the unpressurized chamber where the clutches are bathed with fluid. And again, the hydros contain a total of 5.4oz of ATF for the pump, hoses, and pistons. How do you propose that there's enough fluid to also bathe the clutches? The hydros have to be a closed-loop, high-pressure system in order to operate the pistons with continuously variable pressure. The clutches have to bathe in a sump of fluid that's at atmospheric (or very near that) pressure - you're not going to design two axle seals to withstand 200psi of pressure. The two cannot share 2/3-cup of common ATF fluid. Here's a pic of the hydraulic control system, with the clutch disks not inserted. In it you can see that the hydraulics are self-contained in this assembly whereas the clutches face the large common chamber in the diff.




The large common chamber for all the mechanicals sort of appears to be segmented by a "bulkhead" the cutaway pic I first posted. But that's an illusion created by the webbing at the top of the chamber that they didn't cut away. The eLSD holds 1.6qts of gear oil - 0.4qts more fluid than the plain-jane mechanical LSD. That's because the eLSD needs the extra 0.4qts to bathe the clutch disks. Here's a video from a mechanic who has opened up an eLSD to show us some wear issues: You can clearly see that the clutch disks are covered in gear oil in the beginning. And at the 0:19 mark he actually says the fluid is shared in the clutch area. At 0:32 he points to the piston surface that squeezes the clutches together, and you can see that the area in there where the clutches ride still has lots of gear oil in it, whereas the hydro section behind that is still sealed (he hasn't opened it). At 1:16 he shows us the main part of the housing that the eLSD section bolts to. It's one big chamber, and the eLSD section bolts onto this with the clutches facing into this chamber:



EDIT TO ADD: Here's a document on troubleshooting and servicing the eLSD section of the diff. In it, they explicitly state that one issue is loss of the hydraulic fluid (ATF) due to leakage past the clutch piston seals into the main diff housing. If you see that the piston is behind the clutch pack, then it's clear that if fluid leaks past it where it's not supposed to go, the first place it goes is into the chamber where the clutch pack is. IOW, the hydro fluid is not supposed to be on the clutches! In that document there is also a picture of the axle shaft hole on the clutch-pack (driver's) side of the reassembled unit with no axle inserted. It shows a clear view all the way through to the spider gear shaft in the differential itself:

MSR

MatthewMiller offered all manner of technical evidence/argument, he's clearly correct. Just anecdotally, years as a mechanic for cars that were autocrossed and tracked, I know what gear lube looks like when LSD clutches have spewed material into it. Both of my eLSD C7s dump gear lube full of clutch friction/plate material every time. Also, the lockup behavior in autocross changed when I used Motul, and I ended up switching back to the OEM part number because it seemed too abrupt.

range96

Wow! Very convincing pictures, video and argument. On the previous picture it looks like they are separate. I'm fairly new to the C7 and could be wrong about this after all! I'm still trying to remember where I saw the information that made me think that the clutch pack uses the ATF since I did not personally observe it. If I'm wrong I'll freely admit it and welcome the learning experience.

BTW, I have 3 qts of DEXRON LS Gear Oil (GM Part No 88862624) waiting to be used on my '19 ZR1.

MatthewMiller

I think some of the GM language in its manuals/literature on this topic is slightly confusing. They refer to things like "the eLDS fluid" or "eLSD system," without being clear that they are only referring to the hydraulic controls for the clutches. They consider everything else in the rear housing to be "the differential," and that's filled with "differential fluid." They should refer to the "eLSD control system," or something like that. I wouldn't normally jump into a thread just to correct something I see that's incorrect. However, in this case I think it's important to correct advice being given to others regarding what fluids are acceptable to use in these diffs. Ironically, I had to sell my Corvette (C4) and buy a new Camaro (1LE) in order to have an interest in this thread! I need to replace my fluid, too.