Friction modifier?

[Rich Z]

What the heck does THAT mean?

I'm planning on replacing the fluid in the rearend and recall something being said about adding a "friction modifier" to the lube for the limited slip differentials. Fortunately the Royal Purple stuff already has this included, but it got me to thinking.......

What exactly IS being modified concerning friction? Does it make the lube less slippery or more slippery? If less, then what does this do about the lubrication of the gears and contact surfaces themselves? Don't you WANT the lube to be as slippery as possible to limit or reduce friction and subsequent wear? So does having a limited slip differential mean you have to use a lube that is less than optimum for the wear of the rest of the rearend?