Gear Oil for 12 Bolt Posi

[Ron Sutton]

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.

I'll second what Alocker said.

[AMSOILGUY]

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.

Ok I will clear this up as far as AMSOIL is concerned. If you have a modified engine AMSOIL does not recommend a drain interval. If you have an unmodified performance engine then you can practice a drain interval of 15k with the proper oil and proper oil filter. Yes AMSOIL has a drain interval of 25,000 miles in vehicles that do not have any kind of power adders from the factory and do not have any design issues. Also this 25,000 is only in normal operating conditions. The thing is that 98% of all vehicles on the road operate in severe service not normal operating conditions. Where people go wrong is not every opening the hood during that interval. I remind all people that I work with to check the oil at the normal drain interval. Top it off if needed and unless you are using an AMSOIL filter you MUST do filter changes. Its not a lubricants fault if the owner doesn't check the oil and runs it low and oil temps start going up as the oil disappears. At this point is when you start seeing problems that people will blame on the oil when really its negligence.

[bdelco68]

Just joining and piping in! I have a 396 Elco with 12 bolt 3.73 posi (moser). I've had the car for 22 years-weekender so very few miles/yrs on rear end oil changes. First change was with 75-90w Red Line sync, chattered badly, added GM additive ran down the street and no chattering (fix it).
Second oil change did a Red Line sync NS (no limited slip formula, but added Red Line additive) Red Line said this was the hook-up for posi's. Chatters away even with two tubes of additive.
Dumped that and went to conventional oil Lucas HP 80-90w with GM additive. Super happy- rear end is so quiet now I won't go back to sync (too costly, and to me just not the right formula for older model gears). The conv oil seems to flow better and actually stronger in retaining velocity, even when the rear end gets hot. Chatter-less happy now.
I'm a sync believer for motor oil and power steering, for newer vehicles, since they are made for them. And I also prescribe to the fact that sync motor oil can handle the 7.5k miles range (not 3K or 5K....that's just to often).
But for good old chevy rear ends conv gear oil with or without GM additive (as directed) is my choice now.

[Roadrage David]

Being heavily involved I road racing I can tell you why not to use synthetic oil.
Synthetic oil does not CLING where a mineral oil does!!. in other words a synthetic oil is metal repellent!!. Both synthetic and mineral oils have pro and conns. But when it comes to road racing we always go mineral in engine trans and rear.. oil of choice is Brad Penn racing oil http://www.penngrade1.com/Products/Gear-Lubricants.aspx although a mineral oil is less lasting, it has a better protection and absorbs heat quicker!! and takes on more dirt, and gives of a better anti scuffing. where a synthetic does that job less. hens’ a synthetic oil needs almost double the amount of ZDDP for it to be or give the same protection in high performance oils. for optimal use of zddp which is a anti scuffing, you actually NEED scuffing for it to work!!!. downside is that it won’t last as long, and although oil viscosity is the same labelled in both mineral and synthetic oil a 20w50 mineral has the same thickness and protection as a 10w40 synthetic oil..... There was a time when synthetic oils where dominant but since a couple of years Mineral oils have been improved and are definitely on the comeback and are 50% of the cost compared to a synthetic..

[Barrrf]

Searching for recommendations and found this thread - thought I would copy and paste the info direct from Eaton Differtials Owners Manual under the Lubrication Specifications for the Eaton Posi. Hope this helps anyone else who may search for this thread in the future.

"Quality mineral or synthetic lube with sufficient friction modifier content will ensure the smoothest operating and longest life of your Eaton Posi performance differential. Eaton recommends GM Dexron LS or Castrol Carbon Mod SAF (Synthetic
Axle Fluid) for all Posi applications. If using non-“LS” gear lube, add four ounces of General Motors “Limited Slip Axle Lubricant Additive” per one-and-a-half quarts of gear lube (GM P/N 88900330/10-4003). Additional friction modifier additive may be used to reduce LSD plate “chatter,” but do not exceed eight ounces per one-and-a-half quarts of lube. Other brands of additives may be used. A friction modifier's purpose is to lower the static coefficient of friction (grabbing) within the clutch to match the dynamic friction (sliding) in an effort to reduce the effects of friction transition (stick, slip, chatter). Excessive amounts of a friction modifier can lower both friction values, resulting in reduction of bias ratio (performance) for plate-style limited-slip differentials such as the Eaton Posi. Many brand name gear lubes contain some limited-slip friction modifier in their formulation. If you use such a lube, additional friction modifier additives should be used sparingly to avoid accidental over-saturation resulting in performance loss."

[1BADBET]

I don't think in the conventional vs synthetic argument that one is better than the other per say. It's what the part your using it for was DESIGNED to use. Viscosity, heat dissipation (thermal expansion), wear, clearances, and seal type/ material are all designed into a part AND the fluid they intend to run with it. I mentioned in another thread VW transmissions will NOT function unless you go to a dealer and get the VW brand oil that is specific to the trans. So your best bet is to do whatever the manufacture recommends, I always use the GM stuff when I can as it is always quiet (chatter) and never had any problems with it. I would imagine the gm engineers have their stuff together and a little larger R and D budget over say Lucas whom I've heard spends 80% of their budget on marketing not research lol