What was found
CREW REPORTED A LOSS OF OIL PRESSURE IN LEFT ENGINE DURING CRUISE FLIGHT. CREW REVIEWED THE WITH EMERGENCY ENGINE SHUTDOWN CHECKLIST WHEN OIL PRESSURE DECAYED INTO THE YELLOW ZONE. AS SOON AS THE OIL PRESSURE REACHED THE RED ZONE, THE ENGINE WAS VOLUNTARILY SHUTDOWN. AIRCRAFT RETURNED TO POINT OF DEPARTURE (KMDD). AIRCRAFT LANDED WITHOUT INCIDENT. UPON LANDING IT WAS OBSERVED THAT THE OIL FILL CAP WAS IN PLACE AND PROPERLY ALIGNED. UPON REMOVING THE OIL FILL CAP, IT WAS OBSERVED THAT THE CAP O-RING WAS CUT WITH A PORTION OF THE O-RING MISSING. OIL WAS OBSERVED AROUND THE AREA OF THE OIL FILLER CAP.