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Old 08-05-2013, 08:45 AM
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Will this oil work in my posi?

I have a GM 12 bolt rear end with a posi track diff. I am hoping this oil will fit the bill. I see it says 'limited slip' on it, but maybe newer LSD's are different than old school hardware?

Lucas 85w-140 gear oil
Exceeds service level:
  • API MT-1
  • API GL5
  • MIL-PRF-2105E
  • MACK GO-J
  • PG-2 Limited slip
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Old 08-05-2013, 09:43 AM
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I would use it, and I would still use some friction modifier if you have a plate clutch posi
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Old 08-05-2013, 09:49 AM
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Which differential do you have? It's very important to know this before choosing the oil. If you have the Eaton Posi then this oil will work. You may need to use the additive if it pops. Try it without the additive first. If you have the Auburn then just go ahead and use the additive. In my experience I have found that the Auburn needs much more additive to get it to stop popping. It usually takes 2 bottles of additive. If you have a Truetrac then you need no additive at all.

I wouldn't use a 140 weight oil. The 80W90 or 75W90 is plenty good for these cars. The 140 is for heavy trucks and towing.
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Old 08-05-2013, 09:51 AM
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Quote:
Originally Posted by lmsport View Post
I would use it, and I would still use some friction modifier if you have a plate clutch posi
Thank you. I am about to pop the cover off the diff for the first time. I have no idea what a plate style posi looks like, or what a non-plate style looks like, or what kind I have.

What kind of friction modifier should I buy? Do you have a part number? Can you add too much friction modifier (since this oil apparently has some already)?
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Old 08-05-2013, 09:54 AM
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Only use GM or Ford additive. DO NOT use the additive that the auto parts stores sell. Yes, you can use too much additive. The additive allows the clutches to slip easier. Too much additive will make the differential unlock when you don't want it to. You need just enough additive to keep it from popping.

If you don't know which differential you have then post a picture of it and we can identify it. The Eaton has 4 springs, the Auburn has 5 springs and the Truetrac has no springs.
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Old 08-05-2013, 10:14 AM
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Quote:
Originally Posted by big gear head View Post
Which differential do you have? It's very important to know this before choosing the oil. If you have the Eaton Posi then this oil will work. You may need to use the additive if it pops. Try it without the additive first. If you have the Auburn then just go ahead and use the additive. In my experience I have found that the Auburn needs much more additive to get it to stop popping. It usually takes 2 bottles of additive. If you have a Truetrac then you need no additive at all.

I wouldn't use a 140 weight oil. The 80W90 or 75W90 is plenty good for these cars. The 140 is for heavy trucks and towing.
This is for a 4,000 lb truck. At least that is what I think an 81' Chevy C10 weighs. I don't plan on towing anything (other than my obese buttocks). Will the 140 be OK?

What are criteria makes this oil OK for Eaton's and not the others? The API specs?


Quote:
Originally Posted by big gear head
Only use GM or Ford additive. DO NOT use the additive that the auto parts stores sell. Yes, you can use too much additive. The additive allows the clutches to slip easier. Too much additive will make the differential unlock when you don't want it to. You need just enough additive to keep it from popping.

If you don't know which differential you have then post a picture of it and we can identify it. The Eaton has 4 springs, the Auburn has 5 springs and the Truetrac has no springs.
I am 99.99% sure I have an Eaton. I will post pics!
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Old 08-05-2013, 11:14 AM
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140 will work fine, it's just thicker than you need. It will have a little more drag on it and lighter oil disipates heat better.

The Eaton is a better design than the Auburn. The way the Auburn is made causes it to pop more, especially when it's worn. This oil will work with the others, but if you have an Auburn you will need to use more additive than with the others. If you have the Eaton then it's fine to try this oil without the additive, but if it pops when turning corners you will need to use the additive.
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Old 08-05-2013, 11:47 AM
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There is too much sulpher in the GL5. Eats ferrous metal. Google it. And it a task and half to find GL4 or lower.
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Old 08-05-2013, 12:11 PM
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There is too much sulpher in the GL5. Eats ferrous metal. Google it. And it a task and half to find GL4 or lower.
Is that a typo? If not ferrous metals, then what type of metals are they building power trains out of these days?
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Old 08-05-2013, 01:42 PM
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It attacks yellow metals, brass, bronze, copper... I don't know of any differentilas that are still using any of these metals. The old Eaton Posi had brass thrust washers in it, but all of the newer ones have steel thrust washers. If you have an old Eaton with the brass washers I recommend changing them to the later steel washers, no matter what oil you use.
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Old 08-06-2013, 09:56 AM
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IIRC avoid synthetics with the Truetrack as per Eaton.

peace
Hog
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Old 08-07-2013, 07:11 AM
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GL5 is the newer standard for manual transmissions; essentially the MT1 added to GL5 makes it "yellow metals compliant"

GL4 is the old school rating for vintage gearboxes ie Muncies, T10s, Toploaders, A-833s; axles, transfercases etc.

GL5-MT1 is essentially GL4

That being said, I've had more trouble with the GL5 (MT1 or not) being too slippery for bronze ring transmissions than causing corrosive damage. Personally, we've never seen a ring etched by the lubricant; and we still use bronze backed thrust washers in general production, along with the forged bronze rings.

Always add the oil and try it first, then add a friction modifier if required. If your diff has a vent, clean it. Wear gloves when you pop the cover if you're married.
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Old 08-07-2013, 09:50 AM
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Quote:
Originally Posted by AutoGear View Post
Wear gloves when you pop the cover if you're married.
HA HA, After 15 years in the rear end business and closing my chop a year ago I don't have to listen to "Get away from me. You stink" anymore.

Last edited by big gear head; 08-07-2013 at 09:59 AM.
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