i want to change my rear diff fluid, before i do it i want to make sure of a few things. forst do i need a gasket or is the cover sealed with sealent? second is i am going to use royal purple 75w-90 max gear gear oil. it says it has posi addidative in it, but wantt o make sure i do not need to add any. thanks. truck is an 08FX4 with the facotry 3.73 posi rearend.

 

To seal it use RTV. 75W90 is the wrong fluid. It should be 75W140. Also whether it has the friction modifier or not, I would get a bottle of the Ford friction modifier and use it.

 

75w140 huh, geez that sounds heavy. adding the bottle of ford friction modifier wont be to much? lastly what weight does the front diff take?

 

Your owner's manual has the proper recommendations.

The rear takes full synthetic 75W140 with Ford modifier, the front takes conventional 80W90, but 75W90 synthetic is allowed.

The factory uses RTV, but you may use a gasket if you prefer.

 

sounds good thanks guys. will to much modifier have any bad effects on the clutch pack?

 

No, adding the Ford FM will not be too much. Some have had trouble with those lubes that have FM in them. Adding the Ford's has in many cases fixed their problem.
Note, FM is not needed for the front as it is an open diff.

 

if you put too much friction modifier in, the limited slip wont catch very often. if you use the right amount it will normally. if you use no friction modifier it will catch all the time and work great, but youll be replacing the clutch packs after a while.

i would get one bottle of friction modifier, put it in (after draining the old and putting the diff cover back on) then top it off with full syn 75-140 and your done.

question is what brand should be used? mobil 1 stuff is 20 bucks a quart, im sure fords is that high. wally world sells full syn 74-140 for 10 a quart but who wants to trust that. redline might work but seems to already have friction mod in it so that would be tricky to add friction mod to or trust their amounts.

 

Amsoil Severe Gear...the only way to go.

Also may be worth looking into...I think Ford has spec'd the 75W140 for the front axle now as well. Obviously 75W90 won't hurt a thing. I think they only did this to save cost by buying one fluid.