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