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Anyone using Royal Purple in their rear differential?
If so are you using the RP75x90 with Ford's 4oz bottle of friction modifier? How do you like it?
 
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I use RP but use the 75w140 in my 2011 and 2014 3.7. Along with the Friction modifier. I don't notice a difference over the fluid Ford puts in. No gear noise and still smooth. But it's nice knowing what exact fluid you have where and for how long it will last.
 

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I use RP but use the 75w140 in my 2011 and 2014 3.7. Along with the Friction modifier. I don't notice a difference over the fluid Ford puts in. No gear noise and still smooth. But it's nice knowing what exact fluid you have where and for how long it will last.
So I'm considering switching out to the royal purple in the diff as well. It has the 3.55 lim slip. So I'm curious why are you using the 75w140 instead of 75w-90?

Thanks!!!
 
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That is the recommended weight and such for the 3.7 as well as my area. Coyotes might be different.

I believe Ford revised the recommendations for 8.8s to use 75w140. Any synthetic of choice will be ok. I have yet to have any issues with my 2011 and 2014 3.7. Both run this fluid. The 2014 for about 5 months now.
 

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I think Kevin is confused. For the 11-14, the fill is 75w140. However this is in the 2015 section and 75w85 is the rear end fluid listed in the manual. I'm sure you'd have no trouble with the slightly thicker 75w90.
 
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