As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AIRCRAFT HAD OIL PRESSURE FLUCTUATIONS IN CRUISE AND RETURNED TO BASE. GROUND RUNS CONFIRMED LOW OIL PRESSURE AT IDLE. OIL FILTER WAS CUT OPEN & THE RUBBER FLAPPER VALVE WAS FOUND LOOSE AND MISSING AT 3/8 X 3/8 INCH PIECE. MISSING PART WAS FOUND IN OIL PRESSURE REGULATOR. ALL PARTS OF RUBBER WERE FOUND. OIL FILTER REPLACED AND AIRCRAFT RETURNED TO SERVICE.