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Discussion Starter · #1 ·
The 4.10 gears on my '94 wagon are a little noisier than usual (they like to moan when off the throttle, especially when turning, but are quiet under acceleration when going straight), so I figure I ought to do a fluid swap. The diff runs pretty warm--after a 300-mile highway drive in the summer, the area above the differential in the interior of the car is noticeably warm, and the diff itself is something like 150F according to my laser temp gun.

Should I stick with 75W90?

Or go with 75W140?

I was thinking that the 75W140 would be thicker during the high heat and provide better protection, but then again the thicker fluid is harder to pump, which will generate more heat. Both of the links above include the limited slip modifier.
 

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Depends on posi carrier. Eaton style posi mfg. says specifically NOT to use synthetic gear oil.
Always use 1-2 4oz.bottles of GM posi additive unless gear style like TruTrac. Inexpensive gear sets ,and/or inexperienced installer typically results in noise of some variety. Ranging from barely noticeable to intolerable.
 

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Discussion Starter · #3 ·
Depends on posi carrier. Eaton style posi mfg. says specifically NOT to use synthetic gear oil.
Always use 1-2 4oz.bottles of GM posi additive unless gear style like TruTrac. Inexpensive gear sets ,and/or inexperienced installer typically results in noise of some variety. Ranging from barely noticeable to intolerable.
How do I tell what kind of posi carrier? Why use GM posi additive if the differential fluid says it includes limited slip additive?
 

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You don't know what parts are installed on your car?
Guess you're on you're own then. It really does matter.
I'd sure stay away from synthetics if I were you.
 

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Discussion Starter · #5 ·
You don't know what parts are installed on your car?
Guess you're on you're own then. It really does matter.
I'd sure stay away from synthetics if I were you.
Bought the car more than a decade ago; differential changed out by PO. Last time I did the differential fluid, I did the Mobil1 75W90 with the LS fluid installed. That was.. probably 40K ago. Not sure how many total miles on the 4.10s.

Why avoid synthetics? What could go wrong?
 

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From three posts ago...
Eaton style posi mfg. says specifically NOT to use synthetic gear oil.
Contact them if you really need to know why. I simply took them at their word.
Grown weary of repeating myself ,and you aren't new. Don't you have service manuals?
 

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Discussion Starter · #7 ·
From three posts ago...

Contact them if you really need to know why. I simply took them at their word.
Grown weary of repeating myself ,and you aren't new. Don't you have service manuals?
Can we pretend I am new and dumb, please? LOL
 

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Can we pretend I am new and dumb, please? LOL
OK...."limited slip" diff's do not want synthetic oil. This applies to Eaton or Auburn diff's. Eaton has this in their FAQ 411. Synthetics, over time, do not get along with the material the clutches are made of

Cheap gears or poor install....no oil will make them quieter.

Just use regular dino 80-90 wt with either a 4 oz bottle of the GM "friction modifier" of the Ford LS additive. FWIW my Eaton came with the Ford stuff

I also have 4:10 gears. No noise and no high heat during use but I have a T56....4:10's with a 4L60E...ouch that would be high RPM's on the Fwy...which is more likely the cause of higher diff temps
 

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Pull the cover, and check for lash. You can decrease the lash with shims. Set it without the preload shims, then add the preload shims.
 

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I was always under the impression that the reason synthetics are disallowed for the Posi is because synthetic base stock oils will invariably have a higher viscosity index, which is normally considered a good thing (i.e. less of an absolute increase in viscosity with decreasing temperatures), but these really do need the extra viscosity at low temperature so it behaves like a 80W and not a 75W. Synthetic gear oils will normally be 75W90, 75W140, and now 75W85, whereas dino oils are typically monograde or 80W90, 85W140, etc.
 
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