Thanks to all for the help on my other diff problem. I should have been thinking and asked this as well, but I wasn't. So I didn't.
What oil goes in the diff? Is it different from front to back and the rear being a lsd does it need a friction modifier?
Just getting some conflicting reports so I thought I would ask here and get the right answer.
Its an '02 FX4 4.0l axle code R7
And while I am here.... Best spark plugs?

Thanks

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2002 FX4 4X4

75w90 oil

motorcraft factory replacement plugs.




If you truely have an 02 Fx4 with the 31spline torsen diff, the differential has two tags on it. One is the normal axle information tag. The other is a tag saying "85W140 Synth Only". Or something very similar. Personally, I wouldn't want to consider running that thick of oil in a cold climate....it's just be super thick goo. I'd run 90 weight synthetic. Your rear end, being an 02 Fx4, doesn't require a friction modifier, but I've been told by many, including my axle guy, to run it anyway...even in an open diff. Makes everything slick as hell and happy lubricated gears, are long lasting strong gears.

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'01 XLT disguised as a '03 Fx4 Level II.

Well if I just opened my eyes. Yup, 2 tags. 75 W 140 synth oil only.
Thanks for that. It seems so simple when you know what to look for.
I am sure i speak for all the 'stupid question' asking newbs when I say thanks for the patience and good information.

I'll be back.

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2002 FX4 4X4

Dude...no worries. I totally forgot about the second tag thing up until last week! I've got one of those axles under my truck and in less than a year I totally forgot about it! lol.

Welcome to the site and don't hesitate to ask questions. Of course, searching helps off the bat, but if you can't find, make the thread. lol.

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'01 XLT disguised as a '03 Fx4 Level II.

if you would of used 75w90 oil you would of ruined it. fx4 requires synthetic only of 75w140 beause it has a friction modifier in it.

Nice to know about the other tag. I always wondered how one would know if they had the synthetic oil or not.

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Sgtsandman's Work in Progress
2011 Ranger XLT, 4X4, 4 door Super Cab. 4.0L, 5 Speed M50D-R1HD, 3.73:1, 8.8" open differential, Jason Pace cap
2019 Ford Ranger STX FX4, Super Crew, 2.3L Ecoboost, 10 speed automatic, 3.73:1 with rear locker, Leer 100XR cap