Ethan S. Hawke

So I'm replacing the fluid in my rear diff on my 06 f150. The limited slip is sitting a tiny bit. What is everyone's favorite brand they recommend for the fluid?

bassJAM

For items like diff that don't get changed that often, I like to step up to Amsoil.

Ethan S. Hawke

Do you use any extra friction modifiers to help with the shuttering, or do you just stick to what additives are already in the fluid?

bassJAM

I added the friction modifier. It's been 15 years since I replaced the fluid in a truck with a limited slip, but I believe I used Motocraft brand friction modifier.

Just doing a fluid change might not help, those clutch packs only seem to be good for about 100,000 mi. The good news is you can replace the clutches and make it like new again.

ShirBlackspots

How can you tell when the rear diff is slipping (as I have a limited slip 3.55 rear diff)?

Kkline

I used royal purple 75w 140 plus a bottle of friction modifier for your limited slip. Good time to swap in a larger diff cover too.

Ethan S. Hawke

You can feel a shutter when you turn start driving left or right from a complete stop.

Ethan S. Hawke

Thank you for all the help.

dukedkt442

75w-140 for the rear. I prefer Redline over RP, but I do have the RP in mine. It has the limited slip additive. Chances are you have nothing much left of your LS clutches anyway, though.

ShirBlackspots

I had assumed that was because of the shocks being worn out and were allowing the tires to spin some. My rear pinion seal is worn out and leaking (I do have a replacement, but haven't gotten around to installing it). Do you suppose it could also be because the diff is low on fluid?