What was found
IN CRUISE, MASTER CAUTION, (LOW PRESS R) ILLUMINATED, OIL PRESSURE INDICATOR READ 20 PSI. CHECKLIST INITIATED, ENG WAS SHUTDOWN. UNEVENTFUL LANDING WAS MADE. INVESTIGATION REVEALED THAT A WASHER FOREIGN TO ENG HAD BROKEN THROUGH A SCREEN, ENTERED OIL PUMP, DAMAGEING PUMP. IT IS UNKNOWN HOW WASHER GOT INTO OIL TANK (TANK HAS A SCREEN AT FILLER OPENING). WASHER MIGRATED FROM TANK THROUGH THE OIL LINES TO A SCREEN WHICH IS PART OF THE PUMP (AND CONSEQUENTLY NOT INSPECTED UNTIL PUMP REMOVAL). THE SCREEN FAILED ALLOWING THE WASHER AND PART OF THE FAILED SCREEN TO ENTER THE PUMP AND DAMAGE THE PUMP ELEMENTS. IT IS SUSPECTED THAT THE WASHER MAY HAVE BEEN IN OIL SYS FOR SOMETIME, PERHAPS EVEN SINCE MFG. (K)