Gear Oil for 12 Bolt Posi

[68 SuperRam]

Looking to change rear axle fluid this holiday weekend.

Have Orignal 12 bolt - with a posi (Eaton) 3.73 gears on my 68 Camaro

What weight of oil should I use - was thinking of Royal Purple 75W90 Synthetic. Currently using regular gear oil with the GM additive.



Any thoughts or suggestions?

Thanks

[68lovebuzz]

Personally I would stay away from synthetics. Tend to leak plus moser and I believe eaton advise against synthetics.

[68lovebuzz]

The weight and additive sounds good though

[Schwartz Performance]

75w90 synthetic is fine with no additive.

[rchaskin]

Personally I would stay away from synthetics. Tend to leak plus moser and I believe eaton advise against synthetics.

http://www.eaton.com/Eaton/ProductsS...ials/index.htm

I dont run synthetic in my new Eaton Posi. check the FAQ page.

[Ron Sutton]

http://www.eaton.com/Eaton/ProductsS...ials/index.htm

I dont run synthetic in my new Eaton Posi. check the FAQ page.

I'd like to start an open minded discussion on this, if you guys are interested. Why do you think they do not recommend synthetic oils?

[moreHP]

I do not know the reason for it but a local guy who build lots of diffs doesn't recommend synthetics. He says that regular gear oil works best.

[Ron Sutton]

I do not know the reason for it but a local guy who build lots of diffs doesn't recommend synthetics. He says that regular gear oil works best.

Hi Bob,

With all the humor I can muster up in an Internet post, I say ... "but, why?"

I've been involved in oil tests for an auto manufacturer and found good & bad mineral & synthetic oils ... pros & cons with each ... and find our best gear & bearing life with a certain brand of synthetic gear oil.

Years ago, I low buck road raced with a posi ... overheated it ... added ProBlend to get it to run cooler ... which it did ... but it made the posi clutches slip too much & not engage enough. Many people experience this with certain oils. Some oils cause the clutches to grip too much ... other oils cause the clutches to slip too much ... and the right oils find the happy medium. This can be done with mineral & synthetic based oils.

So I'm still curious why the Eaton engineers say they don't recommend synthetic oils?

Anybody "know for sure" ... why?

[alocker]

Ron,
It sounds like you have answered some of your own questions.

My unprofessional opinion would be this-Allowing the customer to find the happy medium while still honoring a warranty would not be cost effective for the manufacturer. I would also assume they have seen more excessive slip issues with synthetics versus mineral.

I have seen this come up in ATF discussions. Some say DexVI is best but gives poor shift quality (ie slip) but longest life where Type F is gives the best shift quality (ie less slip) but needs to be changed often.

There is also the party that says you should not run synthetic in a motorcycle engine with a oil bath clutch. Some people had issues, I ran it in my roadrace bike and never had an issue.

Soooo, I dont really know why but I think its based on probability of friction coefficient (dependent on lube used).

[minendrews68]

I have a '07 Chev 2500 HD with posi. The owners manual, and the service department at the local Chevrolet dealer both recommend synthetic. I use Mobile 1, and haven't had any problems. One thing I did do that I've never done before was to change the rear end grease. I started to have some chatter in the rear end and was told it was time to change the fluid. Changed it, added a little of the additive that comes in a tube, and haven't had any problems since.

Carl

[Quickboat]

Even running a TrueTrack, Moser did not recommend a synthetic. There are no clutches, but I will break in the gears with mineral and replace it with RedLine Synthetic....insurance.

[BuzzKillian]

I have an eaton posi unit in a 12 bolt rear. I used the recomended Richmond synthetic lube when I changed the gears. When I start out cold the posi is quiet. When it warms up, the clutches chatter in a slow turn.

I swithed to conventional lube (Lucas 90-140 +GM additive) The chatter has gotten better, but has not gone away totally.

After some research I have found that the springs for the clutches have been updated to 800# springs. SO... I'm not sure The chatter will ever go away with rebuilding the unit with a more street friendly configuration.

During my research, I found a couple diferent places that did NOT recomend synthetics for Limited Slip Differentials. I did not ask why... Ron has me wondering now.

[68 SuperRam]

Thanks for all the info - correction the unit that I have is an auburn posi unit - forgot that I had it changed -guess I will stick with regular oil and add the additive -but good points
raised -very informative.

[Rod]

ummm?? the original link about the posi from eaton is for a locker? there are no plates to slip in a locker...there for no slip additive needed and a thick non synthetic will cushion the gears better, also the Detroit tru trac , is a planetary gear setup(no plates also) I run that one and I used synthetic at first and the rear had a hum to it so I removed the synthetic and used a standard gear oil and the rear ran with less noise and cooler

[Schwartz Performance]

Synthetics are not recommended in truetracs, nor is friction modifier as also stated in the link posted before.
Under the Eaton Posi section, it does not say "synthetic not recommended" and we've run it with no issues.

Rod- click the link and scroll down as there is more than just the locker on that page!!

-Dale

[Rod]

Rod- click the link and scroll down as there is more than just the locker on that page!!

-Dale

got it thanks for the forum wedgie!

[AMSOILGUY]

Looking to change rear axle fluid this holiday weekend.

Have Orignal 12 bolt - with a posi (Eaton) 3.73 gears on my 68 Camaro

What weight of oil should I use - was thinking of Royal Purple 75W90 Synthetic. Currently using regular gear oil with the GM additive.

Any thoughts or suggestions?

Thanks

Another option is to use AMSOIL's 75-90 it will protect your unit. I'm offering a deal that really makes it affordable. If you didn't already change and would like pricing let me know.

Personally I would stay away from synthetics. Tend to leak plus moser and I believe eaton advise against synthetics.

Synthetics do not cause leaks. Leaks are a result of a gasket or seal not holding up its end of the deal.

Hi Bob,

With all the humor I can muster up in an Internet post, I say ... "but, why?"

I've been involved in oil tests for an auto manufacturer and found good & bad mineral & synthetic oils ... pros & cons with each ... and find our best gear & bearing life with a certain brand of synthetic gear oil.

Years ago, I low buck road raced with a posi ... overheated it ... added ProBlend to get it to run cooler ... which it did ... but it made the posi clutches slip too much & not engage enough. Many people experience this with certain oils. Some oils cause the clutches to grip too much ... other oils cause the clutches to slip too much ... and the right oils find the happy medium. This can be done with mineral & synthetic based oils.

So I'm still curious why the Eaton engineers say they don't recommend synthetic oils?

Anybody "know for sure" ... why?

Ron,
The problem is that the term synthetic is vague. Which also means that synthetic lubes can be very different in formulations. Maybe I’m the only one but I doubt it. How many times have you heard someone say oil is oil. Well the same thing has happened with synthetics. People hear the term synthetic and assume they are all created equal. The truth is that anybody that has done some research has found this to be not true. So what would make everything a lot easier on everybody would be if the companies would just state a spec that is required. We as the consumer would know what is acceptable rather then just having to take the answer that synthetics are not recommended and wondering why. Especially when you look at a post like this and get both answers and unfortunately the OP has just resorted to using a conventional plus an additive when its very possible he would achieve better results with a synthetic.

[MonzaRacer]

Generally dino works better in vehicle that get into water or mud. Reason being is that it mixes better with water rather than letting water sit outside next to metal causing rust. Most diff builders see this and pass on synthetics. For specific load requirements synthetics DO work better than straight dino oil.
So lets talk oils, the majority are distilled or "cracked" from the crude oil stocks till the base oil is left. They do this with a cracking tower, different petroleum distillates re-condense at different heights in that tower.
THEN this "base" oil is blended with other chemicals, materials to give it the characterisitics it needs, like ZDDP(zinc) helps cams no wear out as flat tappet lifters rub on them, phosphorus, etc. Also specific chemicals to reduce foaming, detergents to clean (so your rings dont stick, or crud build up in lifter galley, etc.
In gear oil there are lots of high pressure lubricating compounds and such, friction modifiers allow the clutch plates to "break" loose "smoother" rather than jerk(chatter). Used to be we used whale oil as its characteristics worked very well. Remember up until the advent of using petroleum products whale oil was preferred lubricant.
Now some say that synthetics are too thin and provide less shock loading over a thicker dino oil.
NOW one fallacy synthetics and blends STILL depend on petroleum. The biggest thing is that true full synthetics, are ENGINEERED to operate at specific loads, heat ranges, other conditions.
Regular oils have irregular molecules where as synthetics are all the same for the majority of them. When designed to provide specific operating parameters they DO perform better in our cars.
Now is a synthetic REQUIRED, not unless the item WAS setup to only operate with it. Like some of the newer SULEV vehicles are using 0w20 synthetic because synthetics are only form of lube that would perform at that low of weight.
Reason for it is less viscosity means less oil in chamber to burn, and less pumping loss of power.
Now do we NEED THEM? No but if you are having lube breakdown or running a road course where oil temps will get high (this is how most oils are made so too high just cracks them further) then changing is a requirement.
Synthetics were developed for aircraft, and sub-zero Alaskan temps.
You need thicker dyno oils to protect heavy loads, but wont work in high altitude or super cold temps.
Now if your daily driving, rare track day at local strip,,,,,good grades ie not thin stuff, will work. I know some very fast Super Pro drag cars that run Coastal gear oil from AutoZone and run fine.
Now want to gain some lost power then synthetics will help regain it. NOW in some instances blends work best, using benefits of both regualr and synthetics without voiding warranties or causing microwelding.
Yes SOME of the above mentioned brands just dont build for the current synthetics.
I know on one larger tree trimming company that had an astounding number of engine/equipment failures after switching too Royal Purple, seen quite a few of this type of issue.
Not to bash but many years ago the Amsoil company was making claims its products would NOT hold up to. We had guy bring in a Ford car, 302 FI, Amsoil sold him on their synthetics, and stupid long oil change times,,,,his car at 23k miles had NO bearing material left and cylinder was worn almost .080" reason being the salesman forgot to recommend oil analysis every so many miles to verify oil life as the engineers used to recommend.
Many people thought synthetics meant it lasted longer,,,which they do, too a point and even under adverse conditions. EVEN RACE TEAMS CHANGE LUBES AT REGULAR INTERVALS.
They also analyze it to see what wearing and adjust additive packages or application lubes.
After recent observation ,and helping a guy that has used Amsoil recently, I am less inclined to bash the brand.
Back too the 302,the guy was told to run 15k to 20k oil changes. Got into it with Amsoil and Ford as engine was supposed to be under warranty. Ford had him sign contract after replacement that he change oil regularly or ZERO warranty and they would sue for past loses.
Synthetics also do NOT mean longer change times, even now the companies are suggesting lower change mileage.
If you use synthetics, pay attention. Most gear companies say run specific breaking procedures in time miles and such. Like Sierra gear wants dino oil and additives for posi for so many miles to get gear/oil temps up and break in the surfaces. then let cool and change oil, then drive another 500 miles, change again then get so many miles on before switching to synthetics.
I do same on street engines, race/drag engines Im not so worried about oil consumption and they are built to somewhat higher standard so I break them in and clean them out with dino oil,,maybe some breaking lubes/additives then after its all set swap oil and filters.
Most of time we run majority of dyno runs on regualr oil and only swap for last few pulls to get rpm baselines and to check what reduced timing does and such.
I have helped build/tune several cars that have adjustable timing capabilities to help with track conditions, or weather, etc.
Honestly, if you dont need $50-$60 in oil,,,why run it.
If your gonna run road courses and have 600,700,800 hp then sure maybe stepping up would help or you experience regualr breakdowns on dino oil then if your sure its oil problem and not oiling system, switch.
Also many of my projects I still try to get better performance and mileage out of so Ill use synthetics where possible.
Lots of people have had much success with synthetic Power Steering oil as they have higher boiling points and better hot wear characteristics. I know in Ford PS units that whine synthetics REALLY shut them up much easier than swapping parts.
Hope this helps.
Good Luck,
Lee

[alocker]

Lee,
Great info above.

I have to say anyone who would buy into 15 to 20k oil change intervals in a high performance engine needs his head examined. Ford got taken on that one.

[Ron Sutton]

Ron,
The problem is that the term synthetic is vague. Which also means that synthetic lubes can be very different in formulations. Maybe I’m the only one but I doubt it. How many times have you heard someone say oil is oil. Well the same thing has happened with synthetics. People hear the term synthetic and assume they are all created equal. The truth is that anybody that has done some research has found this to be not true. So what would make everything a lot easier on everybody would be if the companies would just state a spec that is required. We as the consumer would know what is acceptable rather then just having to take the answer that synthetics are not recommended and wondering why. Especially when you look at a post like this and get both answers and unfortunately the OP has just resorted to using a conventional plus an additive when its very possible he would achieve better results with a synthetic.

Agreed. Thanks for the post.