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Rear Diff Oil and Friction Modifier Question

Big6ft6

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Ok so I just bout 4 quarts of the Mobil 1 Full Synthetic oil to change out my rear diff oil. The label on this stuff has a big red tag that says "limited slip" in and makes a big deal that it is recommended for all standard and "limited slip hypoid differentials" and "meets or exceeds Ford M2C-1924"

Does this mean the friction modifier is already mixed in the oil? Or do I still need to add friction modifier?
 


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swshawaii

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Mobil 1 Synthetic Gear Lube 75W-140 LS contains special friction modifiers designed for limited slip-type differentials. In most applications, the addition of special LS (limited slip) additives is not required. For axles requiring the highest level of limited-slip performance, OEM specific LS additives can be added to this fluid.

You may consider filling it four ounces low, driving, and listening for noise.
Seems easier than removing with a suction pump. Should be OK without it, IMO.

http://www.mobiloil.com/USA-English...il_1_Fully_Synthetic_Gear_Lube_75W-140LS.aspx
 




Big6ft6

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Looking back at my bottles, I used 3 quarts of Valvoline SYNpower, also claims to have frictio modifiers in it, and 1 quart of the Mobil LS (I grabbed 3 at one store and 1 at another, grabbed silver bottles...didn't realize I was getting different brands). 75W-140.

I was under the impression that I didn't require the frictio modifiers, but now after the fluid change...I think I'm hearing rear end noise (only drove around the block)...but who knows I tend to be extra paranoid in these situations.
 




1996BLKBauer

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I used Lucas Oils in my rear end, it also said it is good for limited slip diffs, and that a modifier is not needed. I read that AFTER I put the modifier in, I have no noise though, so I am not concerned.
 








swshawaii

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Ok so I just bout 4 quarts of the Mobil 1 Full Synthetic oil to change out my rear diff oil.
I used 3 quarts of Valvoline SYNpower, also claims to have frictio modifiers in it, and 1 quart of the Mobil LS

That's more than one quart over the recommended fill level for your rear differential.
Owners manual capacity is 5.50 to 5.80 PINTS. Fill 1/4" to 9/16" BELOW the fill hole.
 




SoNic67

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True, 5.80 US pints = 2.9 quarts.
And I wouldn't mix the brands of fluids.
 




Big6ft6

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I only put in about 3.5 quarts I didn't add any friction modifier and I filled until it came out of the fill hole. So maybe that last .5 quart accounted for the 1/4 inch up to the fill hole? So there is only .5 quarts of Mobil 1.

Would having an extra .5 inch of fluid make the rear end louder? I'll pull some out and add the XL-3 (i had picked that up when I bought the oil, but didn't use it when I realized the oil already had it in there).
 




RomeovilleIL

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I have never heard of problems with them being overfilled from the plug. Doubt it would cause any damage since the design is basically a sling to keep everything lubed and there is no pump to create high pressure. I suppose if you pulled the abs sensor off the top and filled from there you could get theoretically get the differential level high enough that it would not cool properly and maybe interfere with the sensor, but since its above the breather hole it would leak out back down to normal.
 




swshawaii

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Your level is correct when you are 1/4" to 9/16" below the the fill plug.
Remove enough fluid to add the friction modifier and it should reduce noise.
As Romeoville said, excess fluid would overflow through the breather vent.
Don't need to make a simple fluid change difficult. Check for leaks, done.
 




bluestream1

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Your oil has the FM already in it no need to add more. FM is not a lubricant so keep it ti a bare min.
 




JAMES209

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Do I need to add a FM to Walmart's SuperTech 75W-140 full synthetic? It states on the label that it can be used to top off LSD's.
 




bluestream1

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yes you will
 




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