Yekoms

1) 150k miles never changed. Should it be changed?
2) What brand fluid? Lucas or the Ford dealer brand?
3) How would I know if truck has limited slip differential (4.2 L XL RWD)?

ANT99F150

1) You should change it, no harm in doing so.

2) Can't help you much with suggestions with fluid, other members can give theirs.

3) There should be a tag on one of the differential cover bolts that has a set of numbers and letters on it. If you have a limited slip it should read 3 L 55 for example. If it's an open diff will read 3 55. The L indicates it has a limited slip.

akdoggie

1) oh yeah
2) Your preference. Some have had problems with Valvoline.
3) The VIN tag on the drivers door pillar. If it's LS, it will have a letter and number (ex:H9) or if it's reg open diff, it will just be numbered. It is on the right lower part of the tag and says AXLE

77Ranger460

I would use the cheapest stuff on the shelf as long as its the correct weight and add some LS additive if I had LS.

kd4gij

I would never use the cheapest oil for anything. Even if I where selling it I would still put good oil in it.

Jbrew

Same here, API certified GF-5 for the crank. Mobile 1 or Amsoil for the rear gear. I had a problem with the Valvoline, just their very cheap in price gear oil, did it all over again with Mobil 1, problem solved. Hate doin shiii twice.

Like AK posted above, your door place card will reveal rear end type.....H9 = LS = 75W -140.

akdoggie

AND XL3 modifier if it is LS

white89gt

^^^^ Yes, make sure you use Motorcraft XL3 only - and add it even if your bottle of gear oil says "limited slip compatible" because it does not work as it should on Ford's LSD, despite the manufacturers claims.

Yekoms

[QUOTE=akdoggie;6160793]AND XL3 modifier if it is LS[/QUOTEi
Its not a limited slip.

77Ranger460

Then, I would buy a couple of bottles of Super Tech gear oil in the right weight at Walmart and drive on.