i bought an after market differental cover and royal purple fluid for it. Do i need to add friction modifier to it? the speed shop says i dont need to because its already in the royal purple fluid.

 

I wouldn't.
Check there web site

what cover? We love them Pics

 

I have heard most synthetic fluids don't need friction modifer. Then I heard some do and some don't not all. Since I heard conflicting things I used it. My take on the deal is adding it has no negative effects so why not use it. Cheap insurance.

 

Synthetic or not doesn't mean anything - the fluid filled in the differential when the truck is new from Ford is synthetic and they add friction modifier.

If Royal Purple says you don't need it, try it - if it chatters or clunks or whatever, add it. It's only $5.00 from Ford.

 

I used mobile 1 with mine and did not add the friction modifier. Mine works great.

 

i got the t/a performance one off ebay. i putting on this wensday with my jlp traction bars before the old town trip

 

All it does is makes the posi-unit slip a little easier around turns. I never use it because a little chattering doesn't bother me, knowing I am getting better posi than if I added it.

 

I agree with Blown347Hatch

 

Do a search for "friction modifier." This has been discussed over and over again for years.

 

Here is a pic of my truck with the JLP rear end cover.