I have a 2003 screw F150 5.4 w/ 3.73 limited slip. The rear end was replaced or rebuilt at the Ford dealership by the previous owner. I have a slow rear axle leak and need to add some fluid. I'm wondering what type of fluid they would have used Synthetic or non and do I need too add friction modifier to top it off? I called the dealership and person I spoke w/ doesn't make me feel confident in their answer. The level is about 1/4"-1/2" below the fill hole

 

Synthetic or non doesn't matter if the viscosity matches the spec. Whether or not you need friction modifier is dependent on if you have an open or limited slip differential, and which one.

 

I thought you weren't supposed to mix Syn or non - Syn lubricants?

 

On flat ground, pull the plug and fill until it's about to come out. Seems like you should be more worried about replacing the blown seal rather than the fluid level.

 

I can't, deer hunting is next week and its gonna have to wait. Is that seal tough to change? I have done the plugs and COP's, brakes and just did the rear u-joint myself but don't want to get into something I can't complete

 

Which seal? By the wheel, or pinion?

 

at the drivers side rear wheel. It leaked for a while and has stopped?? The pinion seal also leaks a little and I'm scared to tackle that one because of setting the new one correctly. I was really tempted when I did the U-Joint tho

 

Not as hard as you may think. Pull the wheels, brakes, diff cover, bolt holding the pinion in, then two c-clips on each axle shaft, slide and hammer for the seals, may as well do both and both bearings while in there. Put everything back and fill up + a bottle of FM from ford. Did it in less and 2 hours taking my time.

 

Did that job a few months back. Easy enough to do. I didnt change the wheel bearings, I thought to myself at the time I would regret that but we shall see.