I've been playing with lunar retrograde orbits in an orbital simulator. The retrograde lunar orbits with apolunes below 40,000 km altitude are pretty stable. They seem resistant to distortion from earth's gravity or the sun's gravity. So I've been thinking of the ceiling for stable LDROs as around 40,000 kilometers.
What is the floor for stable lunar orbits? I am told that if the orbits are too low, they'll be wrecked by mascons.
By stable I mean an orbit that will last a few centuries without station keeping.