Ok, getting ready to service the gear oil in my F450 with Dana S110. Axle code is 8L. Which is 4:88 with limited slip.

Question is what limited slip came in these trucks?

If I mash the go fast pedal, both rear wheels spin. Lift the rear wheels off the ground and spin a wheel, the other spins opposite direction. This tells me it is a gear type limited slip.

Don't want to put friction modifier in if it's a gear type

 

To my knowledge, all Ford LS get the friction modifier. Do you not have an owner's manual to check? You could always call the service department at your local stealership.

 

Not all use a friction modifier. Depends on what unit is used. Found a Ford lubricant chart. The F450's S110 is listed as no friction modifier. Makes some since. Dana makes the housing/axles. Eaton makes the carrier/case. Probably has a Tru-trac. That would explain the way the axle is operating.