Here's a paper on LRO stationkeeping. The fundamental problem is articulated on page 3:
LRO will fly in a mean 50 km mission orbit that, for scientific purposes, would ideally be
a perfectly circular orbit. Due to the lunar non-spherical gravity, however, long term
propagations of low lunar orbits show significant altitude variations and secular drift in
periselene and aposelene altitudes. In fact, this drift will cause the orbit's periselene
altitude to decrease until the spacecraft collides with the surface of the moon. . . . .
A spacecraft in an initially circular polar orbit about the moon at an altitude of 50 km will impact the Moon in approximately 41 days, assuming an altitude of 12 km for impact - the height of the tallest mountains on the moon. At 100 km altitude, the impact is delayed until about 150 days. At 200 km altitude, impact is avoided as the altitude variations are bounded. Therefore, a 200 km orbit is valuable since stationkeeping costs are eliminated. For orbits at 100 km and below, stationkeeping maneuvers are required to maintain altitude control.
The paper goes on to analyse stationkeeping requirements and maneuvers, and concludes:
In its $\Delta$V budget, LRO has 150 m/s allocated to stationkeeping for a one year nominal
mission duration. As demonstrated through analysis, this allocation is sufficient to meet
the five mission constraints for stationkeeping maneuvers.