As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED LOW OIL PRESSURE DURING THE FLIGHT. WE ADJUSTED THE OIL PRESSURE RELIEF VALVE. THE OIL PRESSURE WAS NORMAL AT THE BEGINNING OF THE NEXT FLIGHT THEN OIL PRESSURE DROPPED TO 55 PSI. AFTER REPLACING THE OIL FILTER OIL PRESSURE TURNED BACK TO NORMAL.