Synthetic oil kills trac-loc?

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Most of the good synthetic gear oils like Mobil 1 already contain a limited slip/trac-loc additive package so you don`t need the Ford friction modifier.

I`ve used Mobil 1 75W-90 since the early 90`s with no problem.
 
It depends on the type of diff and what kind of clutch packs are used. When I bought my Eaton posi, there was a huge orange tag on it that said NOT to use synthetic gear oil.
 
How about this thinking: The synthetic arguably is slicker than dino oil right. What is friction modifier? It lowers the coefficient of friction between the clutches, making them bite less hard. Seems that synthetic is helping to achieve the same goal.

How I do synthetic rear end changes: Depending upon the type of fluid used, one might not need Ford modifier. I would swap the fluid in and if the rear chatters, then add the modifier alone. It's easy to add modifier but hard to remove just the modifier. ;)

As you can tell, I dont buy that original argument, as most in here dont.
 
e-mail Castrol and ask them if their 75W-90 Syntec gear oil is compatible with your traction-loc rear end without having to add any friction modifier.

I`ll guess here that they say it is,and doesn`t require the modifier.:shrug:
 
Some posi units, the manufacturer recommends not using synthetic. I don't think castrol syntec is a true synthetic anyway.

You can run synthetic in the stock posi no problems.
 
HISSIN50 said:
How about this thinking: The synthetic arguably is slicker than dino oil right. What is friction modifier? It lowers the coefficient of friction between the clutches, making them bite less hard. Seems that synthetic is helping to achieve the same goal.

Thee is the crux of the arguement right there.......