I am debating on synthetic oil for my rebuilt 12 bolt, with eaton 3 series and 3.73 gears. I have looked at the past posts on this topic. Some loke petrol, some amsoil. Any new thoughts?

Time wont change the info. Sean. Nothing wrong with 100% syn like Amsoil in your Eaton. My new Eaton posi is doing great with Amsoil in it, runs cooler and gets better gas mileage as well. :thumbsup:

I recommend either Amsoil or Lucas.

Anyone use the richmond brand synthetic. They sent me some with my gears

Eaton engineers say their limited slip carriers "perform best" with a petroleum/mineral based gear oil (GL-4), and the 4 oz. bottle of GM (or Ford) friction modifier. See below (scroll down to "Eaton Posi").

http://www.eaton.com/EatonCom/ProductsServices/PerformanceProducts/Products/Differentials/FAQ/CT_128414

There's a 'zone' between driveability and limited slip performance. The engineers know where that is and how it's achieved.

FWIW, Dana and Auburn's engineers also recommend against the use synthetics in their limited slip (posi) carriers. Below are their service manuals:

(see "Important" under "Lubrication" on page 3 below)
http://www2.dana.com/pdf/5323.pdf

(Click on "limited slip" then page 2 of the embedded file in the link below)
http://www.auburngear.com/aftermarket/document_listing.aspx?app=0&id=46613b56-ce27-4e83-b808-9a0c543e09b9&pn=document_categories&returnurl=%2faftermarket%2fdocument_categories.asp x%3fcurrentpage%3d0%26pagesize%3d20

The Dana article you linked to is from 1979, I think gear oils have improved since then and the Auburn article only says that if you dont use the fluid and additive they recommend, that some chattering might occur, a hint to use a lube with the Posi additive in it.

And to keep the playing field level, there is more info. on Synthetics in Eaton gears here:

Many years ago Eaton tested a synthetic fluid in their LSD. The tractive coefficient of the fluid they had tested was apparently too slippery for the LS clutch packs and caused slippage and “chattering”. Eaton thereby made the less than logical assumption that this particular synthetic does not work; therefore ALL synthetics will not work. Because of this, you now see the little yellow cards that come with the LSD units that state “DO NOT USE SYNTHETICS”. Today’s advanced synthetics with friction modifiers and limited slip additive packages are more than sufficient for the high performance LSD. To take it one step further, AMSOIL has come to find that our gear lubes possess frictional charachteristics that are equal to, and in some cases outperform the Eaton factory fill bulk fluid. Eaton does not want to spend the money to run tests on all of the various synthetics that will work in the LSD. Therefore they are stubbornly standing by their old obsolete recommendation.

When it comes to the issue of warranty it is AMSOIL’s corporate stance that our gear lube WILL work in the Eaton LSD. Should you ever have a gear lube related failure with these units, while using AMSOIL gear lube, and Eaton decides not to honor their warranty as a result, AMSOIL will repair and/or replace the damaged unit. You have nothing to lose and higher performance with a much more stable base stock to gain.

AMSOIL Gear lube WILL offer maximum performance and protection in the Eaton LSD Amsoil Tech Dept. 2003.

Those stubborn engineers and their silly R&D.

If syns performed better, ran cooler, increased service intervals, increased HP, extended the life of the carrier, increased mileage, and (most important) reduced warranty claims, the Eaton engineers (and Dana/Spicer and Auburn, the three giants in the industry) would recommend synthetics for their limited slip carriers. But they don't. In fact they go out of their way to tell us to not use synthetics.

If you purchased a new Eaton posi today, it would come with a tag attached that says:

"Important Application Note, It is strongly recommended the Eaton Posi LSD you have purchased be installed using mineral based 80w90 Gear Oil and 4 oz. of friction modifier (GM#1052358). The Eaton Posi LSD is not recommended to be used with synthetic gear oils."

It's said above that 'Eaton doesn't want to spend the money to test all the available synthetics...' That helps me make my point. Why would anybody pour an untested recipe into their differential not knowing the subsequent results? One of three things will happen:

1) It will run great
2) It will bump and chatter
3) The limited slip effectiveness will be reduced.

If you run the recommended lube and friction modifier, one of one thing will happen; it will run great!

Yes, the service manuals cited above have been around for years as have our carriers. But as you can see, these are the latest bulletins currently posted on the web-sites of the manufacturers. Apparently there's been no compelling reason to supercede them.

There are cars today, for which a synthetic is recommended; newer Vipers for instance. But rest assured, the carrier's clutches were designed for use with synthetics (and probably a particular synthetic; not just any ZYZ brand).

I'm sure the shareholders of Amsoil, Royal Purple, Red Line, etc, would all like us to purchase as much product as possible. In that light, their recommendations don't hold up to the recommendations of the unbiased engineers at Eaton who design, R&D, and study failures. I doubt anybody could show a credible recommendation for synthetics for our old carriers that doesn't come from the marketing dept of the company selling synthetics. I've presented three unbiased sources (above) that recommend against their use. If you want to see a fourth, review the owner's manaul for your Camaro.

Amsoil's warranty to replace the carrier after a failure if their lube is at fault, is very hollow. Amsoil gets to decide who/what caused the failure. I'm guessing they've never paid a claim from this warranty. Please read (especially number 5):

http://www.amsoil.com/lit/g1363.pdf

Not saying you'll experience a failure, but that warranty is no reason to run Amsoil.

Lastly, count how many trillions of trouble-free miles that have been logged on the millions of Eaton limited slip carriers in the last 50 years running on mineral lube and GM additive.