C2 340 HP rear diff fluid

[Richard S.]

I would appreciate if anyone has information on present day rear diff fluid and additives for my 63 vett. I am looking at changing my rear diff fluid and not sure what is available and correct for the application. The rear is a posi-3:70 ratio. Probably less than 2000 miles on present fluid but many years old.
Thanks
Rich Sheridan

[Harry S.]

Re: C2 340 HP rear diff fluid

This is what I use in my 63's


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[Richard S.]

Re: C2 340 HP rear diff fluid

Thanks Harry the info, I did see that on the shelf at Advanced Auto. I will give it a try.

Rich Sheridan

[Duke W.]

Re: C2 340 HP rear diff fluid

Differential oil doesn't need to be changed on a time basis like engine oil, but Positraction axles should have a fluid change about every 30K miles to remove the clutch wear particles that are held in suspension and eventually drop out to the bottom forming a sludge.

Do you have a 1963 Corvette Shop Manual? The original Mil-L-2105D oil specified therein was superseded by SAE 80W-90 GL-5 gear oil in the seventies, available at ANY auto parts store and many big box stores. Brand make no difference. It's a generic product. So just make sure the bottle has the API "donut" and SAE 80W-90 GL-5.

I do not recommend heavier weight oils or synthetic oils. Heavier weight oils are suitable for heavy duty trucks, and Corvette differentials in normal road service never get hot enough to benefit from the more consistent viscosity of synthetics at extreme temperatures.

All clutch types limited slip differentials need the same friction modifier. You don't have to buy it from GM. The NAPA part number is NCB 4285V and any other parts store should be able to cross reference that number to whatever brand they carry. They are all the same stuff. One four ounce bottle should be sufficient for Positraction axles.

The axle oil capacity is in the service manual, owner's manual, and AMA specs.

Duke

[Richard S.]

Re: C2 340 HP rear diff fluid

Duke

Thanks for the feedback. I just stopped by my local auto store and was looking at the various oils. I noticed that even the SAE80W-90 with GL-5 rating indicated use only for added top-off not full replacement of my diff oil. Is there other SAE 80W-90 GL-5 rated oils that do not have this notation? Does the friction modifier allow for full fill up with the SAE 80W-90 GL-5 oil mentioned above?
Thanks again
Rich Sheridan

[Duke W.]

Re: C2 340 HP rear diff fluid

I've never heard of such a statement on a SAE 80W-90 GL-5 bottle. Do more shopping. I don't understand your last question.

Duke

[Richard S.]

Re: C2 340 HP rear diff fluid

OK. When you look at the details on the back of the oil 80w-90 GL-5 rating, What I see stated on the label only used for top off not for filling an empty diff. And the question if the printed label information is correct what additive if any makes the stated GL rated oil usable for an empty diff?

Rich Sheridan

[Joe L.]

Re: C2 340 HP rear diff fluid

I've never heard of such a statement on a SAE 80W-90 GL-5 bottle. Do more shopping. I don't understand your last question.

Duke

Duke------


I agree. I don't understand such an instruction, at all.

[Gary R.]

Re: C2 340 HP rear diff fluid

Rich
You can use the Lucas 85-140 or 80/90 wt. I use and recommend the 85-140 in the diff's I build and there are literally 100's of them out there using it without any issue. For non ncrs builds I add a drain plug to make it 10 minute oil change. I change the oil out about 8-10k miles on my cars.

The additive is required and the present stuff may require 2 bottles. I run 2 in fresh rebuilt diff's with tuned posi's. The last time I compared the NAPA and GM were about the same price, at least around here. I use the GM stuff and always have.

[Duke W.]

Re: C2 340 HP rear diff fluid

The instruction sheet says to use "mineral based 80W90 gear oil." That's SAE 80W-90 GL-5. The Lucas 85W-140 is "synthetic" base.

Duke

[Richard S.]

Re: C2 340 HP rear diff fluid

Thank you Gary for your product suggestion and as related to your experience. Sounds like a good way to go. Just a comment here, I seemed to have not worded my question to others with any clarity. Perhaps I can try rewording the question and at least get my thought across. Since the fluids I saw had a notation on the directions that indicated the fluid was for top off only not a complete fill was the additive slip diff modifier indicated above the missing chemical that would make the 80W-90 GL-5 rating acceptable for a complete fill of the diff? Or are there other 80W-90 GL-5 rated fluids available that do not post that stated notation. I have not checked with a GM dealer to see if I can obtain the products noted above. But plan to next week.
Thanks again
Rich Sheridan

[Duke W.]

Re: C2 340 HP rear diff fluid

As I previously stated, I have never seen such a statement on a GL-5 bottle, find it incomprehensible, and suggested you look at brands. What brand did you look at.? How about a photo of the verbiage. I'm going to Walmart today and will check out their SAE 80W-90 GL-5 products.

The limited slip additive is required to prevent clutch chatter in clutch they LSDs. It has no relationship to the statement you said was on the bottle.

Mil-L-2105 and later SAE 80W-90 was FACTORY FILL at least through the end of C3 production and most likely C4 and beyond, and I expect that current solid axle pickups and SUVs are the same.

Duke

[Richard S.]

Re: C2 340 HP rear diff fluid

Duke

The two I remember seeing were Fram and Lucus. I did see one other brand but do not remember the name. Thanks for the detail on the additive use. I will be away for the weekend but next week I will check and see if I find other brands without the limitation and take a snap shot of the detail I saw on those containers mentioned.
Rich

[Gary R.]

Re: C2 340 HP rear diff fluid

The product I recommended and used for the past 20 or more years is Lucas 85-140 #10042 -qt. non synthetic. Lucas does have a 75-140 synthetic gear oil, #10121 that is not what I ever refer to. I even confirmed with Lucas who should know their own product.

The new Eaton posi and Yukons use fiber coated clutches another band aid approach to end posi chatter. Solid steels are all I use and what Eaton used in the 65-79's although the 72-79 were the weaker snowflake junk. The fiber smudges nicely to end chatter, your oil comes out black with it.

GM gear oil can be used if you like but you need additive.

I don't know what that top off statement is but all you need are 2 qts of gear oil and 2 bottles of the gm additive. Suck out as much as you can, again the reason I tap a drain hole, and replace it with the 2 bottles of additive and fill till it just weeps out the fill hole and you are done. If you want to take it to a parking lot and side load it doing slow figure 8's you can, but driving will do the same thing. No need to overthink this, it's a simple maintenance function. You can find the GM additive at any dealership, some parts stores, Summit or ebay. I stick with what has worked in my diff's built for 200 - 900hp and change it at the 8-10kmile range although I'm sure some let it go a lot longer.

NOTE I never use preload spring pack on my posi builds. Less wear, no chatter, still has posi action. When the GM part number changed about 10 years ago I first noticed a posi chatter with a tuned posi. At the time, the previous additive did not require 2 bottles but ever since I add them and that works without issue with the posi.

[Duke W.]

Re: C2 340 HP rear diff fluid

I visited my local Walmart today to look at their gear oil products. The only brand they had was their SuperTech house brand. It was available in a 80W-90 GL-5 mineral oil base, same spec synthetic base, and a 75W-140 GL-5 that was probably synthetic.

The label of the 80W-90 GL-5 mineral base said something on the order of "...for topoff and refill of transmissions, axles... that require a GL-5 specification oil..."

Further along the label states that it's suitable for positive traction axles or some such verbiage, but that doesn't mean it has the friction modifier, and I'm 99.99 percent certain that it does not, so for refill of Positraction axles you stiil need at least one four ounce bottle of the additive.

This product is the current and improved version of the Mil-L-2105 that was factory fill in our cars. It's what I would use and what I recommend to others... about five bucks a quart.

I also buy their SuperTech 15W-40 CK-4/SN engine oil and use it in my vintage cars because it's usually, but not always, less expensive than the national brands.

Duke