As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOTS REPORTED ENGINE NOT MAKING FULL POWER AND OIL PRESSURE LOW. MAINTENANCE FOUND A PIECE OF CLIP UNDER THE OIL PRESSURE RELIEF VALVE. THE RETAINING CLIP FOR THE VERNATHERM VALVE FOUND TO BE THE SOURCE. FOUR NEW VERNOTHERM VALVES INSTALLED. (X)