There seems to be a subconscious idea they(friction modifiers) physically help grip. Almost as though they were pumice powder!
In fact, they reduce friction. So - why are they used in items where friction is a part of operation? When you have two rotating or sliding items meeting surfaces at differing speeds, there is (naturally) frictional drag, despite the oil film. This creates localised or flash temperatures which are pretty darned high - enough to scorch or damage an unmodified oil - a sign of the heat causing unwanted outcomes is the brown glazing; tarry deposits from elements of degraded oil that have glued themselves to the surface.
Ultimately, it is temperature which damages fluids like PAS fluid or ATF, or LHM. By reducing the incidence or severity of temperature spikes, friction modifiers preserve the fluid qualities appreciably longer. The increased purity of full synth oils/fluids means that either tolerances can be improved, service intervals extended, or duty specifications increased because although they also degrade, it result in less "tarry" deposit forming than extracted mineral oils.
