Have a 2017 screw fx4 3.5 eco. I am wanting to replace rear diff lube with Royal Purple 75x140 as I have done in all vehicles, lower units, atv that I have had with great results. My question is that my axle code says "L9". For 3.55 locking rear diff. My confusion is do I still have a limited slip with this since i have to engage locker? Simply stated, do I need friction modifier when doing this or do I not? I cannot find answer, just L9 locking rear. I do not know enough about rear diffs to know more than this. would like to get this done asap. Thanks in advance.

 

Following...

 

The owner's manual makes no mention of adding a friction modifier. It only specifies:

Rear axle fluid: Motorcraft® SAE 75W-85 Premium Synthetic Hypoid Gear Lubricant XY-75W85-QL
Specification: WSS-M2C942-A

I would think if a friction modifier for a LSD were necessary, it would say to use one. But I'm not a Ford tech!

 

no. the add in fluid is for when you have a clutch pack = lsd. you don't have that = no extra fluid

 

The factory fluid is already synthetic. Is there lots to be gained by changing it to another synthetic fluid?

 

Be careful. Changing the diff oil to a non specified product could give Ford a reason to void your warranty in the event of a failure. Might want to wait until it's out of warranty.

 

That would be my question as well. Is it due for replacement? How many miles?

 

The manual says 150,000 miles before any diff fluid service. Since they use a premium grade full synthetic I don't see any reason to change it.

edit: I have had many vehicals with 200k+ on original lube and never had any trouble, not even the hint of a abnormal sound.

 

You have an open diff with an elocker. There is no limited slip. No need to change your rear diff fluid. Esp if it's a 2017. Ford already uses a synthetic fluid.

 

one reason to change fluid is if you submerged the pumpkin in water... otherwise, no need