Speculate, in terms of the microscopic surface irregularities, about why µk< µs, and what effect a lubricant has on the coefficient of friction.

Any surface, when observed under a sufficiently powerful microscope can be seen to have a lot of irregularities (Refer to the attached figure). When two such surfaces come into contact with each other, given adequate amount of time, the irregularities can interlock quite well with one another. Thus, in the case of Static Friction, because the surfaces are not moving relative to each other, the irregularities interlock very well, thus requiring a greater amount of force to dislodge one object from the other.

On the other hand, when an object is moving, the irregularities are not given sufficient time to interlock well with one another and hence, lesser force is required to dislodge one object from the other.

Under such a scenario, we can intuitively understand that when a lubricant is put on a surface, it would occupy the spaces left by the irregularities, thus filling them and not giving a chance for another object's irregularities to interlock. Hence, we see this as a great reduction in the overall friction force.