73, Dark Blue 454

From above: "Other than Eaton, the others really don't come out strongly against or for synthetics, they just indicate that chatter may occur,.."

"Other than Eaton"?..the company who designed and manufactures our LS carriers?..R&D's, studies failures, processes warranty claims, etc.

Eaton's recommendation alone, should end all discussion.

But, did anybody bother to call Strange Engineering, or Moser Engineering, or Tom's Differentials and ask which type lube they recommend for their differential builds equipped with an Eaton posi, and which type lube will void their warranty? Their phone numbers are posted above. The three are very respected giants in the industry and each happens to be an Eaton dealer. (Of course they are,..Eaton makes the best LS carriers).

Like I said, pour whatever you want into your differential. If you use a synthetic, one of the three things will happen:

1) It will run great
2) It will bump and chatter
3) The limited-slip (posi-traction) effectiveness will be reduced because of the more slippery synthetic

If you run the recommended conventional lube and 4 oz. bottle GM additive, one of one thing will happen:

1) It will run great

Also, I could post another two dozen or more sources, in addition to the half dozen I listed above, that would recommend against using a synthetic gear oil. I'd like to see one credible recommendation, that doesn't come from the maker or seller of synthetic lubes, that recommends the use of synthetic gear oils with our Eaton posi's. Just one.

Lastly, just because it's more advanced, doesn't mean it should be used on our old rigs. DexCool and Dot-5 come to mind.

wan2run

This argument has long been debated. I have run across it on the forums I frequent in the past.

From what I recall on the Eaton discussion specifically is that they tested one kind of synthetic oil a while back and discovered it was "too slippery" and caused the clutch packs to chatter. They supposedly didn't have the time/resources to test all synthetics and came to the conclusion that all synthetics must be this way and therefore decided against recommending them. Funny thing is, nowadays the factory fill in the Chevy trucks with Eaton LS rearends is synthetic.

My Nissan Frontier with a Dana rearend also comes with a synthetic 75w-140 fill from the factory.

Now I realize that our 40 year old diffs are different than newer ones, but food for thought....

wcsinx

Ain't much changed

75 Vette

I used synthetic and chattering came back. Here is something to think about: when your C3 differential was invented synthetic had not. I drained my synthetics and put the old stuff back in and everything is fine now.

iokepakai

Using Royal Purple Max Gear Oil.Can't get oil shipped to Hawaii anymore without paying through the A$$.NAPA has it here so I tried it and seems to be fine.No need for friction modifier it's added in already.Pumped mine out with a hand pump then pumped the new oil in.Not an ez task on jack stands.Good luck.....

redvet68

That's what I used for my 68 in 2004 and has been just fine ever since.

Tim H

Myself I would just use Valvoline 85/90 and the real GM posi lube.
I don't know if it applies but people use synthetic oil in their 4 speeds and then the synchronizes wont work right and you grind gears.

marshal135

I went and got the AC Delco brand LS additive as rec. on this forum and 3 qts of 85/90 at Napa.
Sucked out the old and pumped in the new till it dripped out the fill hole.
Buttoned it up and drove it in circles clockwise and counter several times to move it around. No chattering out of mine.
Its not a job that gets done often so just do it and forget about it.
No need to pay all that $ for hi-speed fluid if you are not hammering the rear.
Marshal

bashcraft

I've been using Pennzoil gear lube in the rear for years and never had a problem. It already has the LS additive in it.

I also have synthetic gear lube in my M-20. It's worked fine for 5 years now.

jim-81

Seriously??? use what the manufacturer recommends. Thank you 73 Dark Blue

63mako

I used this in my last 2 rebuilt differentials as well as a few unrebuilt units that i changed the lube in with zero issues or chatter. No additive needed.

C66 Racing

All synthetics are not made from the same basestock (there are three different basestock Groups that in the U.S. are allowed to be labeled as synthetic), and they all have different levels of friction modifier, ranging from none to lots. Thus I feel that it is not accurate to blame synthetics in general on diff chatter. New C6s come with synthetic and almost all of them experience diff chatter within a few 10s of thousands of miles. Those that change over to the AMSOIL universally report the chatter goes away and doesn't come back - similar to 63Mako's experience above.

Though a little dated now and an AMSOIL sponsored study, I think this is still an interesting read for those that want to see how much variance there is from one diff fluid to another:
AMSOIL Gear Lube “White Paper” (2 MB pdf file)

jb78L-82

I have used Mobil 1 75W-90 synthetic with no addtives since the early 90's and just recently changed it again with Mobil 1 with zero issues-no chatter, whining, improper limited differential slippage etc. I think that 20 years of using the synthetic has convinced me that all is good with synthetics. Do what you are comfortable with using but based on the barage of folks that use synthetics with limited slip differentials for the rear ends from the C3 era, synthetics appear to have no downside versus conventional gear oil!

pws69