Remember, a Friction Modifier (FM) can be a friction reducer, a friction increaser, or one that controls friction in a specified manner. In an engine, we want to reduce kinetic friction to increase fuel mileage. In a CVT type AT we want a friction increaser so the chain/belt/pulley system can "get a grip." In an Automatic Transmission or Limited Slip (LS) Differential, we want a controlled and specific type of friction. The frictional characteristics we are discussing here is called Kinetic Friction, Dynamic Friction or Sliding Friction, a special kind of friction. Dynamic friction is a friction that changes its "coefficient of friction" as two surfaces that are in relative motion come into or are in contact as in an AT or in LS differential clutch plates. Recall that AT clutch plates have alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. The steel plate is splined on the outside, where it locks to the clutch housing. The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches are energized at any given moment. In AT's and LS Differentials, we want the fluid to create a specific dynamic friction coefficient (dependent upon the clutch materials used) such that we have smooth engagement and disengagement, so we don't have shudder or slippage. Shudder and slippage cause increased frictional material wear and increased heat. It is this complex package of frictional modifier chemical compounds found in ATFs and LS additives that is important for smooth operation. Again, in an engine, we primarily want friction reduction. In an AT or LS Differential, we want controlled friction modification called Mu(V) in the literature. Mu is the Coefficient of Friction and V is the relative velocity of rotating or sliding machine elements. In special friction test machines, the resulting curve of Mu(V) gives us the resulting Dynamic Friction Coefficient and it tells us how the friction coefficient value varies with the relative speeds of components. The fluid chemistry must assist in providing a specific friction versus velocity relationship for the AT or LS Differential. There is a phenomenon commonly called "stick-slip" or "dynamic frictional vibration" and manifests itself as "shudder" or low speed vibration in the vehicle. Using friction modifiers in the ATF or differential fluids prevents this shudder.