View Full Version : Synthetic gear lube? problems?

06-18-2005, 06:36 AM
I have a Moser 12 bolt with Eaton posi, 3.42:1 gears with c-clip eliminator axles, the Moser instructions say NOT to use synthetic gear oil, but the Eaton instructions say that you should use synthetic oil..., plus I have to add the limited slip additive. Can anyone shed any light on why I may or may not want to use synthetic oil in there? My preference is to use synthetic, but if there is a good, solid reason why I should not, please enlighten me.

06-18-2005, 06:50 AM
Eaton built the posi, Eaton says use synthetic. I would use synthetic. However, there might be an issue with the axel bearings/ c-clip eliminator setup. Contact Moser and ask "Why?"

06-23-2005, 08:11 AM
I'm not sure why the Eaton brochure said to use synthtic but don't. Use some normal mineral oil with a bottle of additive. Synthetic lube can cause chatter. Low speed chatter will most likely develop at some point with the synthetic. I'm a technician at Eaton that has worked on this issue in the past and it is ongoing. Mineral oil with additive is the best oil for the job. Synthetic isn't going to cause a failure but it can cause excessive noise.

07-04-2005, 11:48 AM
I just bought some synthetic gear oil for my dif with an Eaton posi. I should take it back now eh?

07-09-2005, 05:53 PM
I have tried and used just about every oil that has been on the market in the last 20+ years.... and with specific use of the Eaton limited slip, (both OE circa middle 60's and the late model units (there is a big difference between the two)). But, there is no reason that you could not use a full synthetic lube with the the older, solid steel clutch pack units or the newer laminant/fiberous clutch packs. The newer clutch packs however can be a little sensitive to the amount, or lack of, a friction modifier. Some syn's even have their own fric mod built into the base oil. For reference, the better syn's with an intergral fric mod are Torco, Amsoil and LE. All of which I would recommend for use with an Eaton limited slip...... but remember, a new or tuned posi may require more fric mod to operate without clutch chatter. NOT ALL synethics are created equal..... and I wont run synthetic in a differential unless it is from one of the manufactures I listed above.... **for street use**. In a race car, where the differential lube will be changed more than your underwear, I might even consider a product or two from RedLine..... but in my opinion, even their differential products are too thin and dont have the film pressure strength for use in a hypoid gear assembly. And I definately wouldnt use it in a street car........

The best thing I have found for use with an Eaton limited slip differential, in a non-syn, is a parafin based oil from Schaffer. It still requires an additional friction modifier and my first choice of that would be the old Ford friction modifier (which I dont think is even available anymore), but it too is parafin based and mixes well.

Now, as for a NON Synthetic, I have been using Schaffer strait weights with a suspended moly in differentials for near 16 years without any oil related failure or brakedown. This includes differentials in Land speed record events, Top Fuel D+F/C, Pro Stock, Trophy Trucks, SCCA Trans-Am, IMSA..... Ambulances and Fire trucks..... street cars etc etc. If you are/were directed to use a non-syn lube, I would highly recommend the lubericants from this company.

For normal every day street cars I use a strait 140wt and an additional 4oz bottle friction modifier. Sometimes more, sometimes less.... but on an average, 4oz will do just fine. Some times, you will need to drive the car around for a while, so that the oil and friction modifier has a chance to work into the clutch packs....

If you run a full synthetic, run something with quality. I have even heard from some Eaton engineers, off the record, they dont have any problems with Torco or Amsoil..... just keep a bottle of friction modifier handy if you have to add an additional ounce or two in case you experience clutch chatter.

07-10-2005, 02:02 PM
interesting info. why not use red line for the street? i know a lot of the open track guys with fourth gens have a hard time finding any gear lube that can handle the tempurates even red line. ive never heard of an issue with red line on the streets. is this just with eaton rears or any clutch type posi? thanks

07-10-2005, 03:50 PM
No, it is not related to any Eaton product.

Unless they are running the newer heavy weight (140 wt at a min) 'Shockproof' .... the oil will not "climb" and it is way too thin for an 'impact load' environment. It pors like water at room temperature.... and it acts even thinner when at operating temperature. Hypoid gearsets dont like splash oiling....

The main reason people have issues with finding an oil that will handle elevated temperatures associated with performance driving.... is they dont pay attention to the viscosity required for, just that. I am sure some of them are even running a 75-90..... when they need to be running at a minimum... a strait weight 140+. Synethic or not..... I myself, have even been known to use an ISO 250 wt in some street cars, just because they make 800+ hp and are driven pretty hard... on the street no less.

Give their differentials time. I can guarantee the bearings in their differentials are showing signs of micro-spalling and radial styrations.... which is the very beginning of surface failures related to tapered roller or tapered needle bearings.

07-10-2005, 06:14 PM
oh they are destroying the rears. this is even with most of them using the 80-140 redline has. it seems to be with even the shock proof oil or which ever is supposed to be race only.

i have regular redline 75-90 in the back of my formula now. i honestly dont beat on the car. its a daily driver more than anything else. so far no issues. eventually a T2R will ride back there.

thanks for the info.

Ralph LoGrasso
07-10-2005, 06:17 PM
Are there any problems or negative side effects with running such a high weight gear oil if the car is mostly street driven? Will it not get hot enough? I was going to replace the rearend oil in my '01 with regular mobil 1 75-90, but i'm thinking of going with a higher weight now?

07-11-2005, 04:48 AM
Basically at the moment if you don't mind chatter run synthetic with modifier and if you do then run a good grade mineral oil and a bottle of modifier. Theres a very good chance the carbon woven pattern on the discs will make noise. We have been using carbon for years now and synthetic usually makes noise. I'm more of a secondary guy on this noise/ls testing. I do the gear/case durability testing but the guys are in the same lab so I know what they are doing and I know mineral oil is the best choice. Believe me, since January we have been doing thousands of hours of testing to isolate the exact cause/cure of this noise nothing has been determined yet. It is established that mineral/modifier is quite and synthetic/modifier will most likely develop chatter.

07-22-2005, 09:53 PM
Tom, are you sure on your weights? The guy who did my rear end(no jokes please) told me to use regular old valvoline 80/90 non syn. Are you telling me I need a 140 weight?? Sounds pretty damn heavy to me.

07-25-2005, 10:46 AM
Did I stutt ttt tt ter ?

Yes I am sure on weights. I use strait 140wt in everything I have built since 1993.... (with no oil related breakdowns or related failures) with the exception of a differential that is being used in a cold environment (which would get a strait 90wt) and my occasional use of an ISO 250wt. in heavy chassis, land speed record, SCORE off road and HD usage in industrial services.

Big boat anchor chassis, that weight over 3500lbs, that have lots of power and torque or that have requirement to fend off heavy shock loads..... NEED a heavier old from the get go. I can give the phone numbers of some of the top diff builders in the country...... if you want to hear it from some one else beside me.

07-25-2005, 11:09 AM
okay i'll take some phone numbers

07-26-2005, 05:43 AM
I have a 10 bolt with an Eaton posi and stock GM axles. I've been using Mobil 1 synthetic gear lube with GM Posi additive for probably 6,000 miles now without a problem. No chatter, and the posi works perfectly.

07-26-2005, 07:26 PM
You'll have to call me, cuz I ant posting the numbers I have for who you need to talk to.

07-26-2005, 07:46 PM
okay post your phone number.... J/K i will call moser/currie in the morning and see what they say. I am down to change the oil, just want to make sure, otherwise if i have a problem the guy will say it was my fault for not using the oil he recommended. Also would a thicker fluid help a small pinion leak?

07-26-2005, 08:21 PM
Moser and Currie ?? The two most likely not to have an actual clue....... call some real differential shops instead. Here I'll give you the short list:

Mark Williams (http://www.markwilliams.com/)
Toms Differential (Tom's Differential's)
Sommers Brothers
Pro Gear (http://www.progeardifferentials.com/service.html)

I'd call Mark at Pro Gear first, 858-571-1158. He has more differential experience than the two you listed, combined. Not to mention, he's not too cool to talk to you either......

07-26-2005, 08:33 PM
I know, i just threw those 2 names out there because i knew it woudl bother you! Did you have something to do with the invention of rearend fluid also?? :fart: