Maybe I'm wrong here and if so someone jump in and tell me. I thought anything left in lunar orbit would eventually fall to the moon like the LEM upper stages did. Atmospheric drag is not a factor obviously but due to orbital perturbations caused by Earth. Park it at L-5.
There are a couple of "frozen orbits" that are stable around the moon, generally at inclinations of 27 degrees, 50 degrees, 79 degrees and 86 degrees. An object on one of those orbits will remain more or less undisturbed for years with minimal stationkeeping.