Just changed the diff fluid (front and rear) on my 01 Screw. I figured after 120,000 miles or so it couldn't hurt. Planning on doing the same to the expedition (75k on the clock) and changing the fluid in both transfer cases. The owners manual sates that the diffs are considered lubed for life. Has anyone noticed any benefit to changing the fluid? Better fuel economy? Anything?

Also I thought it odd that the 04 Expedition calls for 75W-90 in both front and rear diffs, but my 01 is 75W-90 and 75W-140 respectively.

 

Don't know of any savings or benefits other than maybe if you had some moister before [boat ramp water] the fluid change now you don't.
Did you add friction modifier?

 

No. I used the Mobil 1 which was for a LS diff. Aside from that I have open diffs. :/

 

I think the consensus here is that the Royal Purple is the only stuff you do not have to add the friction modifier to as it seems to have enough in it already. Every other brand, I believe (someone let me know if that is not the case), you should add a 4 ounce bottle. You can get it from Ford.

 

Do I really need it? I have open diffs and haven't heard any noises out of it yet.

 

I changed mine out to Royal Purple and I think I still had to add the additive.

 

With open diff you do not need friction modifier. With the diff fluid change you will probably not see any noticable difference (fuel milage) but it is good preventative maintenance.

 

the official ford spec is that the only time the diff fluid is to be changed is if the axle is submerged underwater. friction modifier is not needed for open differentials.

 

Have had the truck up to the bottom of the doors in water before. Not looking for better mileage, but wouldn't gripe about it if I got it. LOL

 

so have I... i was just listing ford's recommendation. BTW, if you manage to get water lodged in your differential, you can really tear the axle components up.

 

I changed to Royal Purple last summer and now I hear the clutches rubbing when I turn.
So it may still need the additive.

 

These trucks came into the marketplace with different specs & configurations. Consult your owners manual & maintenance schedule. You will need friction modifier if you have limited slip.

 

I changed my RP out to Mobil 1 because I thought I felt some shudder. Now few hundred miles later I really feel a shudder even though they claim to have modifier in it. I'm going to put in the Ford mod. this weekend and will report back.