I can answer your question on how much oil it takes to fill a D36 rear.. I did mine last night and it used 1- 7oz bottle of limited slip fluid and 1 1/2 quarts of gear oil. I also didn't use synthetic, I used Valvoline 80-90 oil. I have read on various sites not to use synthetic in the older C4 vettes and I didn't want to take a chance....WW

 

I may be wrong but in 86(my vettes year) was there synthetic oil of any kind? It was probably cheaper to use the conventional oil...

 

Yup. I replaced mine myself using Mobil 1 Synthetic rear diff lube and the GM axle additive. I used a 1/4 inch drill-powered pump from Harbor Freight Tools to suck all the old fluid out of the housing.

 

Use a name brand dino Gear Oil. I used Castrol 75W-90.

 

 

AMSOIL Severe Gear Extreme Pressure Synthetic 75w90 (stock code SVGQT)

Can't quote prices online, but the above AMSOIL fluid is about a 3rd of the cost you've listed for the GM fluid. I've been using this fluid for years in my 02 Z06 on the track without any failures. Note that this fluid contains limited slip additive and adding extra additive is not normally needed.

 

It's your car and your money, fill it with whatever you want. I'm sure the shareholders of Amsoil, Royal Purple, Redline, etc, would like to sell as much product as possible.

But if the engineers at Chevy (and Dana) recommend mineral based GL-5 lube for the differential, that's all I need to know. If through R&D and testing, Chevy thought using a synthetic would perform better and reduce warranty expenses, they'd have filled 'em with synthetic. Chevy wasn't adverse to the use of synthetics lubes at the time (my 96 came with Mobil-1 factory fill in the engine), but I'm guessing the clutches in the posi aren't designed for the more slippery synthetics.

Sometimes we hear that "chatter" is reduced with the use of synthetics but I submit clutches are simply slipping and the limited slip (positraction) funtion and it's performance is reduced with the use of syns.

Also, I have confidence engineers who designed our differentials and carriers know the exact amount of additive needed to keep the clutches in the 'zone' between driveability and limited-slip performance.

It's crazy when I see folks ask if they need to add the GM additive to the synthetic they just poured into their diff. Who knows? Now you're playing chemist, or you're trusting the chemists at Amsoil, Royal, Redline, etc, vs. the engineers, R&D, and testing at Chevrolet.

Lastly FWIW, it's no secret synthetics will leak before mineral/conventional lubes.

My opinion (and Chevrolet's).

 

Wow, that's really bold of you. My FSM simply says "Rear Axle - Gear lubricant SAE 80w-90 GL-5 (GM part # abc, xyz, or equivalent) and gear lubricant additive (GM part # 123 or equivalent)".

From this your surmise:

- Only conventional oils should be used
- Synthetics may make the diff sound like it is working better, but really it is working incorrectly
- Your pronouncements based on no more than an oil weight and a part # mean Chevrolet backs up your opinion 100%.

I obviously am deficient in reading between the lines, as I see nothing anywhere indicating a conventional oil is the only choice.

Since you mentioned the dangers of "playing chemist", do you have some NOS store of GM gear oil? The part #'s mentioned in my FSM are no longer valid, and I doubt the oils they spec'd 18 years ago for my diff are even still made. Or maybe you don't change yours at all, as after all GM had no service interval for gear oil....

 

One thing that the OP did not mention is what sort of use the car will get. For general street use, the factory recommended gear oil and the GM posi additive will work just fine.

There would be no need for a synthetic gear oil in a different weight unless he wants to spend extra money. In a way, 73 Dark Blue 454 is correct in his statement regarding what the GM engineers did in testing. A GL-5 80-W90 and the GM posi additive has been used in Corvette differentials for many years.

Now, if the car gets subjected to high performance use like drag racing, HPDE's, or similar activity, then it could be reasonable that a synthetic may be a better choice. But people that subject their cars to abuse like you would see in racing would change gear oil much more frequently compared to somebody that uses his/her car as a DD or a Sunday cruiser.

As far as GM part numbers for fluids, most of the fluids were more off the shelf than some GM super-secret formula. If you go to a GM dealership for a differential oil change, the chances are good that the oil used will be from a roll-around bulk container that is no different from is used in most any shop.

It could be Pennzoil, Chevron, or any other brand where it's available in the typical 30 gallon cans. It will meet the GM spec for gear oil for a particular application.

So, my $.02 is to use a good quality 80W-90 GL-5 gear oil and the GM additive. If you want to spend more on a synthetic with an additive or some other oil, then go ahead. For general street driving, the 80W-90 works fine. Go racing or beat on the car, then consider something else.

 

I believe the recommended GL-5 lube from GM is still available under part number 12345977. The limited slip additive is part number 1052358.

Also, please read page 3 under the word "important" from an old maintenance manual (linked below) to verify Dana's recommendation for limited slip lubricaiton.

http://www2.dana.com/pdf/5323.pdf

Lastly, pre-C4, Eaton provided limited slip carriers for decades for millions of GM and other carmakers from the 50's through the 70's and beyond. They go out of their way to recommend mineral / petroleum lube (GL-4 with additive) for their limited slip carriers for the "best" and "optimum performance". This is from their site (see section on 'Eaton Posi'):

http://www.eaton.com/EatonCom/Produc.../FAQ/CT_128414

(BTW, in the link above, you'll note that synthetics are NOT to be used with their newer carriers, the TrueTrac and the Detroit Locker).

If you've purchesed a new Eaton posi carrier lately as a replacement in your older Vette, you saw it had a green tag attached that specifically said to NOT use synthetic lubricants.

I'm banking that the engineers at Chevy, Dana and Eaton know what their doing.

FWIW

 

That Dana doc, while interesting, does not support your contention. It says some manufacturers may spec an additive to the diff oil. Which GM does, and which has nothing to do with whether the oil you use is synthetic or not.

It does however counter one of your side points, in that it says "some otherwise good lubricants do not preclude "chatter" and may need an additive". This in fact supports, rather than refutes, the claim some make that with M1 or Redline oil, that the GM additive is no longer needed. In fact, if your limited slip does not chatter or bump, it would suggest no additive is needed regardless of the lubricant you choose.

Neither the engineers at Chevy, nor at Dana, are suggesting specifically not to use synthetic oils. Which was the point you made initially that I asked for clarification on. So now, you've looped in Eaton to the mix because that's the only company you could find who makes such a suggestion?? You dismissed recommendations from oil companies that make differential lubes already. In spite of the fact that an oil company makes the lubes GM specs. Yet you feel Eaton has some trustworthy advice on how to care for your Dana differential...

 

If it is not the part number recommended in your FSM or owner's manual, it is not the same oil. It may be a different oil that GM says is "equivalent", but I would bet they didn't have a lot of engineers in on that, nor do a lot of testing with C4's before making that claim.

 

I really don't see where anyone said a conventional oil would not be a suitable choice? What I saw was the OP quoting a $130 pricetag for the "GM" way of doing things, and an inquiry as to a cheaper way. Any of these synthetics will still be worlds cheaper than what his dealer was charging.

From there, a contention was made that synthetic oils were bad for your diff, and that GM agrees. Which is what changed the direction of the thread.

 

Ok, I've supported my contention with documentation. Your turn. Without using marketing materials from the makers of synthetic gear oils, find me a credible source that says that the limited slip clutches for our Danas were designed for use with either synthetics or petroleum based lube. In fact, find any limited-slip unit who's manufacturer doesn't recommend one or the other. Again, research limited slip (or positraction) units.

Do I underestand that after reading the Dana doc above, you believe we should add the friction modifier to a synthetic? Wow. How much? 4 oz? Do you know how much modifier is already in the synthetic (if any)? How will we know if the LS clutches are "in the zone" between driveabilty and peak limited slip performance? Some diffs on some cars take 3-4 quarts of lube, some as little 1.25. I'm guessing if the friction modifier is already added to a quart of synthetic, a big diff case gets a lot of friction modifier and small case gets a little. Tell me how the makers of your synthetic knows exactly how much lube was to be used in your Vette and exactly how much modifier to add to their lube recipe?

Through R&D, the engineers at Chevy and Dana already know the answer.

Also, you keep bringing up the FSM. What does it recommend? Petro or synthetic? In fact your Corvette owner's manaul has the same recommendation as the FSM for petro based lube plus the friction modifier.

And I'll ask again, if synthetics performed better and reduced warranty expenses, why didin't Chevy recommend it?

Lastly, it's not unusal for GM to change or supercede part numbers. Trust me, you can still get the recommended GL-5 lube at your Chevy parts counter. I'm sure it's same GL-5 that came with your C4 originally.

 

Found this at a GM truck site food for thought.

"Document Number : 9986115
Publication Date : 2003-12-01 00:00:00.0
Status : CURR (Current MB)
Organization : General Motors North America
1 Scope This specification describes an axle lubricant to replace previous lubricant 9985290 and 9985182. This axle lubricant, suitable for car and truck axles, exhibits improved fuel efficiency, lower axle temperature operation, and improved thermal stability. "
Notice that is say NOTHING about being a "synthetic"; being "synthetic" is not part of the engineering spec, it has to do with the base stock chosen for the lube, that's all. The goal is for a more fuel-efficient friendly fluid, and one that last's longer in the diff, but it DOES NOT mean that warranty will be suspended if you use a previous generation fluid. 9986115 replaces 9985290, that's all. And BOTH are GL-5 API rated gear lubricants.

Further, read this:
"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.
[Comment from Castrol:] Thank you for contacting Castrol regarding Syntec 75W-90 GL-5 gear oil in your Volvo differentials with an Eaton Automatic Lock. You may use SYNTEC gear oil with full confidence.
[Comment from Mobil:] Mobil 1 Synthetic Gear Lubricant meets the GL-3, GL-4 and GL-5 API ratings. If the manufacturer requires the use of a GL-4 rated lubricant ONLY, then you should not use Mobil 1 Synthetic Gear Lubricant. "

Now you will note that the above quote is from Eaton Corp., the manufacturer of our differentials in our trucks. Here, there is specific reference to the fact that they show both conventional AND synthetic fluids meet the engineering standards for the Eaton differentials. There is specific reference to GL-5 fluids; note that I underlined the reference.

You see, GM spec'd out the 9986115 "synthetic" fluid (what we all call grape juice) because it was a FUEL ECONOMY issue, not a warranty issue. This "synthetic" fluid allows for a longer service use, before it has to be changed. But ANY GL-5 Fluid will work and that's STRAIGHT FROM THE MANUFACTURER. There are even references from both Castrol and Mobil that their fluids meet the GL-5 standard as well.

By the way, do you think GM is the only manufacturer to spec out a "synthetic" fluid for differentials? Ford and Chrysler do it too, depending on application. And their specs are numbered differently, of course, but they ALL reference GL-x rated fluids for their differentials. The SAE (Society of Automotive Engineers) and the API (American Petroleum Institute) work together to develop lubricant standards. GM just has internal spec's that reference certain performance criteria, referring to lubricant standards, NOT engineering them. GL-x fluids aren't just for differentials; they are used in many gearboxes in industrial applications too.

"SYNTHETIC" FLUIDS ARE ABOUT EXTENDING FLUID USE, NOT LEVELS OF MINIMUM ENGINEERING REQUIREMENTS REGARDING WARRANTY PROTECTION. ANY FLUID THAT MEETS GL-5 IS OK FOR THE DIFFERENTIALS IN OUR TRUCKS. You can use ANY GL-5 fluid, add the appropriate friction modifer, and get a fluid that meets the specs.

GM just puts in 9986115 (the "grape juice") so that is lasts longer, not because it protects better. It will not evaporate quite as quickly compared to a "dino" lube, and it allows for slightly better fuel economy. If you read the header from the GM engineering spec, it's not even specific to trucks; IT'S ACCEPTABLE FOR ANY CAR OR TRUCK AXLE, AND IT SAY'S NOTHING ABOUT LOCKING DIFFERENTIALS. In fact, the original date I found for spec 9986115 was in 1997, predating the GMT-800 and GMT-900 trucks!

So that is way I stated, and have now proved, that if you're going to change your differential fluid every year, or even probably less than 25k miles per change, "synthetic" fluids and their associated higher costs are NOT NECESSARY!

GM does not "require" a "synthetic" fluid, they "require" a fluid meeting engineering spec 9986115. Any GL-5 rated fluid meets that spec regarding axle lubricant protection properties. The "synthetic" attribute is just an added bonus for better fuel economy, and longer lasting service life.

Here is a link where you can purchase the GM engineering spec:
http://auto.ihs.com/document/abstract/KCMJEBAAAAAAAAAA

Now - I've done all the foot work; and I've even paid for the materials. But because of copyright restrictions, I cannot post direct links to the purchased engineering specs. You want the engieering specs? Then you should pay for them, too.

 

And C6's require synth why?

http://www.c6registry.com/Technical/06vetteom.pdf

 

Just to add to the confusion... In a 77 C3, totally unrelated, I know, I encountered a noisy rear end. It would make a grinding noise on turns after a good long run. Changing the gear oil, using the GM stuff, did not change the condition. Using Royal Purple did cure the condition. No ill effects after 10k miles. Speculate and say what you will, Royal Purple worked for me. I was skeptical at first because GM gear lube is supposed to be the best, I didn't expect Royal Purple to be any good, boy was I wrong.

 

Are we discussing new cars and trucks or our old C4's? I agree, synthetics are better if the clutch's friction linings were designed for synthetics as some newer cars are; no arguement here.

But if the friction linings were designed for petro lubes, then petro lubes would work best according the the manufacturers.

Go back to the Eaton site. Every carrier product listed says to NOT use synthetics except the E-Locker which is what I suppose you're referring. The E-locker, I believe, is used on newer trucks.

Obvioulsy, synthetics have their applications and petro lubes have theirs.

Also, I have no idea why we're even discussing GM's lube # 9986115. That's used on newer Vettes and other applications. For our C4's Chevrolet recommends 12345977, a petro based lube.

Dana's Maintenance Manual, your FSM for your Vette, and even the your owner's manual, all recommend the petro lube.

And again, if synthetics performed better, reduced warranty claims, and increased gas mileage (thanks for that), why didn't GM recommend it for our Danas?

 

Though this is an AMSOIL commissioned test (so feel free to through out the AMSOIL fluid's results), the results are a pretty interesting read on the multitude of trade-offs required for a diff lube.
AMSOIL Gear Lube “White Paper” (2 MB pdf file)

Based on the wide variance shown by both synthetic and conventional diff lubes in this test, I feel that it is a little inappropriate to draw broad conclusions applicable to either synthetic or conventional diff lubes. One brand of either is not the same as another brand.

 

I believe I've quite clearly addressed your "supporting" documents. Please provide a scan of your FSM or owners manual where it specifically says only a petroleum-based oil should be used. The burden of proof is on you dude, you made the contention that GM specifically recommends against synthetic oil.

Or admit your opinions are your own and you are not the mouthpiece for GM engineers.

Or I guess keep making pinhead arguments about how GM would have only used "the best" products for our cars in the first place, but don't be surprised if it doesn't win too many people over. Especially owners of GM products...