As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT, FLIGHT CREW GOT A RT ENGINE LOW OIL PRESSURE WARNING AND MASTER CAUTION. CREW RETURNED AIRCRAFT IMMEDIATELY TO BASE. UPON TROUBLE SHOOTING IT WAS DISCOVERED THAT A SMALL WASHER WAS LODGED IN THE RT ENGINE OIL PRESSURE RELIEF VALVE.