well make sure to drive it hard for the next week and let us know if any reliability issues occur

 

if your installers know what they are doing, they shouldnt whine

 

So, after reading the instructions on doing it yourself, I'm inclined to just get Ford to do it since I'm gonna take back for the whine anyway. Question is - will using this stuff void the warranty?

 

only for not being the right weight

 

The recommended fluid is synthetic 80W140, GL5. What the definition of synthetic is up for grabs. Anything labelled otherwise is a gamble with complying with your stated warrantee requirements. Note that heavier viscosity fluids will actually INCREASE your rear-end's temperature.

 

What about just adding XL3 friction modifier? I know tha has been said to help the graon on turning for some people. Does it help with mild whine? Als, is it in the diff right from the factory? I'm asking because I just got a new rear diff and it wine s abit. If it's not in there by default, I think I'd try tthe XL3 first before the redline.

 

I think the owner's maunul says 75W140 but 80W140 is the same viscosity and the low temp diff from 75W to 80W is probobly smalland omly matters if your in a cold area

 

Adding friction modifier does help quiet down the clutch packs in the diff, if that is what is making noise. (There's a TSB out for that, by the way). Clutch pack noise is evident when turning, especially sharp turns at low speed. However, in this situation it is a band-aid fix. Adding extra modifier just lets the clutches slip more. This makes your "posi' less "posi".

The modifier will NOT affect gear whine.

Gear whine is evident at higher speeds. Generally it gets louder the faster you are traveling. It is also evident all the time, not just when turning.

By the way, I've got about 60 miles on the car since the oil change. Still, no whine to speak of. I have been driving hard and listening for it.

 

I ordered 3 qts of the stuff and also the red line friction modifier. Are you saying I should not add the modifier?

 

I did not check regarding the modifier becasue I am using the Detroit Truetrac diff, which does not require an additive.

However, I am fairly sure that all of the Red Line gear oils already have friction modifier added from the factory. I don't think you need to add any. If you are in doubt, call Red Line and ask them.