I will be changing the rear diff fluid on my 2004 Lariat with the factory limited slip. I have the Ford friction modifier but the gear oil I bought (Valvoline synthetic)
has friction modifier in it. I did not realize this when I bought it. Should I also add the Ford friction modifier when I change the diff fluid?

 

its up to you, if you add it, extra wont hurt anything

 

I would. And did the same in mine. Smell is horrid, but it really works.

 

i will never forget when i ordered some of that on a stock order and it leaked all over the place in the box, i almost lost my breakfast...

 

Thank you. I was planning on adding the Ford friction modifier just to be safe but I didn't know if there would create an adverse problem.
My garage smells like a skunk with the bottle sealed. I'll be glad to get the diff fluid changed.

 

You can put the bottles of additive in a zip lock bag or large jar with a lid.

As said, adding it won't hurt anything, I've had to dose the rear in an old Chevelle I had a couple times to quiet it down while turning until I fixed it.

 

yes. I had my new/sealed bottle double-bagged in zip locks… could still smell it if walking nearby that shelf/area.

 

Add the Ford stuff
If you don't, the rear end will chatter (even with the additive in the fluid already)
Don't be surprised if you need more than 1 bottle

 

Yep... I had to drain some fresh gear oil and add some more friction modifier to the bottle I put in to quite down my 3;73 LS.

 

do you mean that net net, you run two bottles?

I’ve been running one and working well, but i in talking with a mechanic (who I think is experienced and good), he mentioned that he always uses two. Said he has better results and less “comebacks”. (And also said no downside other than cost)