flushing out rear end

[mikeku91]

Im replacing my pinion seal and wondering whats the best axle oil i can put in the rear.

[letitsno]

Amsoil will lower operating temp. of axle.

[NYC F-350]

75W140 synthetic diff oil. If you have a limited slip, before you purchase the "friction modifier" check the label of the oil you buy. Some of them come with the modifier as part of the formulation. If it does not state that the fricition modifier is already in it, it probably isn't.
Start with adding 4-6 ounces of modifier, drive the truck for a couple hundred miles. On a long trip where you know the rear diff oil is up to temp, find a parking lot. Come to a full stop and cut the wheels to one direction (doesn't matter which). Slowly roll into the throttle. If you feel a "chatter" add one more ounce. Drive it like this for a few hundred miles and repeat the test. Never add more than 8 ounces of the modifier. If you still get chatter, your limited slip clutch pack needs to be rebuilt.

[mikeku91]

What brand(s) of oil comes with the "friction modifier"?

[NYC F-350]

Check the label to be sure, but I think Mobil1 comes with it in there.

[mikeku91]

Okay thanks a ton!

[PGreenSVT]

Mobil 1 does for sure but you may need more. Try first without adding and if the clutches chatter add 2 oz. If they still chatter add the other 2 oz.