PLT OF AN SMA SEL DIVERT LANDED AFTER OBSERVING LOWER THAN NORMAL OIL PRESSURE INDICATION.

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IN 1100 HRS IN THIS AIRPLANE; THE OIL PRESSURE NEEDLE IN CRUISE; WITH OIL WARM; 'COVERED UP THE 5' OF THE 50 LB INDICATOR. AFTER CLBOUT FROM FALMOUTH; 5B6; I LEVELED OFF AT 2500 FT. I THEN LOOKED AT THE OIL PRESSURE AND COULD 'SEE THE 5.' I LOOKED AT IT FOR A FEW MINS FROM ALL ANGLES; PINCHED MYSELF; NOTICED IT WAS IN THE GREEN ARC; BUT DEFINITELY LOWER. I DECIDED TO MAKE A PRECAUTIONARY LNDG IN NEW BEDFORD; AND I CONTACTED PROVIDENCE APCH ON 133.85 FOR HELP WITH THIS UNPLANNED LNDG. AFTER 3 UNANSWERED CALLS THEY FINALLY ANSWERED; REQUESTING THAT I STAND BY. SINCE I FIGURED THAT THE OIL PRESSURE COULD CONTINUE TO DECLINE AT ANY MIN; I DECLARED AN EMER. THE LNDG WAS UNEVENTFUL. ON THE GND; A FULL PWR RUN-UP SHOWED; ONCE AGAIN; COMPLETELY NORMAL OIL PRESSURE. BUT I TOOK THE NIGHT WATCHMAN'S OFFER OF A RIDE TO THE HOTEL. THE NEXT MORNING; THE MECH AND I CUT OPEN THE OIL FILTER -- IT WAS CLEAN. AFTER SEVERAL RUN-UPS WITH NO MORE ANOMALIES; AND IN DAYLIGHT; I CONTINUED THE FLT WITHOUT INCIDENT. I TOLD MY BOSS; WHO WOULD HAVE TO STAND IN FOR ME AT MY MORNING MEETING; THAT I RAN A HIGHER PRICED AIRLINE THAN ACR X'S. SHE UNDERSTOOD. NOTE: MECH'S OPINION IS THAT THE INCIDENT WAS CAUSED BY A PIECE OF CRUD INTERFERING WITH THE OIL PRESSURE RELIEF VALVE.