BE-40 CREW HAD ERRATIC; AND THEN LOW OIL PRESSURE INDICATION.

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I WAS THE PNF IN THE R SEAT OF A POSITIONING LEG. WE HAD JUST COMPLETED OUR 'TREND MONITORING' PAPERWORK; ABOUT 50 MINS INTO A 60 MIN FLT TO SNA. WE WERE JUST ISSUED A DSCNT WHEN WE NOTICED THE R OIL PRESSURE GAUGE WAS VIBRATING IN ABOUT 5 PSI INCREMENTS IN ITS NORMAL POS OF 80 PSI. WE HAD NO OTHER INDICATIONS OF ANYTHING ABNORMAL. WE DOUBLECHKED THAT THE R OIL PRESSURE LOW ANNUNCIATOR LIGHT TESTED PROPERLY. IT WAS FINE. WE THEN REFERRED TO THE EMER CHKLIST TO BE READY FOR A LOW OIL PRESSURE SIT IF ONE SHOULD ARISE. THE OIL PRESSURE GAUGE CONTINUED TO VIBRATE OVER THE NEXT SEVERAL MINS SHOWING A SLOW DECREASE IN PRESSURE. AS THE CHKLIST INSTRUCTS; WHEN THE PRESSURE GETS TO BTWN 40-60 PSI; THE THRUST LEVERS WERE REDUCED TO A LOWER SETTING. IF THE PRESSURE CONTINUES TO DECREASE BELOW 40 PSI; WHICH IT DID; THE CHKLIST INSTRUCTS TO PERFORM A PRECAUTIONARY ENG SHUTDOWN. WE COMPLETED THE SHUTDOWN AND ASKED FOR VECTORS TO VNY. VNY WAS DETERMINED TO BE THE CLOSEST SUITABLE ARPT WITH ENOUGH RWY (CONSIDERING A PARTIAL FLAPS LNDG MAY BE APPROPRIATE (PER THE CHKLIST)); AND HIGH ENOUGH WX (WE WERE ABOVE AN OVCST WITH NO VISIBLE BREAKS) TO ALLOW ACCEPTABLE SAFETY. I COLLECTED THE APPROPRIATE ARPT CHARTS AND ADVISED THE PF THAT ATIS WAS CURRENT (2000 FT OVCST; 10 MI VISIBILITY; LIGHT WINDS). WE BRIEFED AND SET UP THE SINGLE ENG ILS APCH AND FELT COMFORTABLE IN OUR PROCS. WE WERE THEN TURNED OVER TO THE LAST SOCAL FREQ THAT WOULD TURN US OVER TO VNY TWR. WE ADVISED THIS CTLR THAT 'WE HAD TO PERFORM A PRECAUTIONARY ENG SHUTDOWN; AND WERE GOING TO FLY THE APCH SINGLE ENG. WE DID NOT REQUIRE ANY PRIORITY HANDLING OR EMER SVCS AT THIS TIME; BUT JUST WANTED TO KEEP ATC ADVISED OF OUR SIT.' THEY CONFIRMED OUR RADIO CALL; ASKED 'FOR OUR PURPOSES; WHICH ENG WAS SHUT DOWN?' I REPLIED 'THE R ENG.' THEY TURNED US OVER TO VNY TWR WHO CLRED US TO LAND. APCH AND LNDG WERE UNEVENTFUL. WE EXITED THE RWY AND TAXIED TO A MAINT FACILITY.