Does friction modifier need to be used in LSD's?
 

Swepco 201 appears to be a given. My transmission still shifts well when the clutch is properly adjusted and I'd like to keep it that way.

"Friction Modifier (FM) is slippery stuff. The FM is in place to reduce LSD clutch chatter upon engagement. The amount needed is relative to the application and the age of the differential. In full race cars, you want the clutch to lock up nice and strong while not being so concerned with a little chatter. In high mileage (or worn) LSD cars, the clutches will be worn and need less FM to attain the desired effect. The downside, or catch-22, is that the syncros don't need or even like the FM. In most cars this is not a problem because the differential and transmission are separated and do not share the same lubricant. Our transaxled P-cars do share, so we must prepare for that.

FM options:

FM needs to be used in LSD cars, we know that. It's the amount that is variable. As a general rule, what comes out of the bottle is sufficient for most applications. Some folks will mix FM stuff with non-FM stuff (75W90NS and 75W90) until the desired effect of "no chatter" is attained... the reason for this being again, that syncros don't like FM."

Swepco 201 does not need an additional FM with a 915 gearbox w/ LSD.

It was my understanding that most full synthetic lube does not require a FM, but I recently bought some Mobil One from the local Ghetto Zone, and it is labeled LS, (for use with LS diffs). This was for a rear differential in my rockcrawler so it didn't make any difference to me.

I have some Amsoil that will be going in my 225k 915 (with factory LSD), and I am not planning on adding a FM. And yes, I have heard that 201 does not require a FM.