The friction modifier is used to prevent chatter (a stick-slip condition) in the differential clutches (that are there to get the inside and outside rear wheels to turn at the same speed.) In a slow turn into the parking space, there isn't a lot of driveshaft torque generated and the clutch pack doesn't get a lot pressure to try and get the two halfshafts to come to the same speed. With minimum pressure, there will be slip in the clutch, and to keep it feeling smooth, the friction modifier is used. With no friction modifier, the rear wheels will feel like they "hop" much like if you've ever left a four wheel drive system engaged and make a sharp turn on dry pavement.
With more driveshaft torque applied, the clutch pack begins to lock up, making both halfshafts rotate at the same speed. In low speed high torque application, we consequently have both rear wheels lose traction with the road surface; not just because the torque applied exceeded the grip capabilities of the tires, but also because both rear wheels cannot spin at the same speed in a turn - one or the other has to slip. The engineering term for this is "doing a donut."
The friction modifier used is an additive selected not just to provide the smooth slow speed slipping, but also to interfere the least (or not at all) with the action of other surface active materials, such as anti-wear additives. You wouldn't want to add friction modifiers (that may have little anti-wear performance) if they compete for the metal surface and don't allow enough anti-wear additives to attach to the metal. That said, I think Mopar recommends one bottle of friction modifier for a variety of differentials that probably have different oil charge sizes, so there is a large margin of safety. And friction modifiers, like all additives, do "wear out" and a recharge (if you have a high mileage vehicle that chatters in low speed turns) is a reasonable thing to do.
ab, I'm not coming up with a reason to think you have to change the fluid sooner if you put more in.
