How far does oil absorption drop after treatment — and when is that a problem?

Usually a clear drop, and the size of it depends on agent type and coverage. For most formulas that drop is the main reason to pay for treatment.

The mechanism: the agent occupies active sites on the surface and lowers surface energy, so oil no longer wicks into pores and surface crevices. On porous powders it is most pronounced, because pore mouths are partly sealed.

Three consequences follow. Viscosity falls, so the same solids load handles thinner. You can push in more powder — pigment or UV filter — without it turning to paste. And skin feel improves, with less dryness and drag.

The catch is the same property working against you. If oil control is the product's function, as in an oil-control loose powder, a treated grade can weaken it, and the untreated version may be the better choice. Measure oil absorption on your own samples before and after, using the standard method, rather than relying on the datasheet figure.