BE99 CREW HAD LOW OIL PRESSURE IN #2 ENG IN ZLA CLASS E AIRSPACE.

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CRUISE PHASE OF FLT WHILE SCANNING INSTS; I NOTICED THE R ENG OIL PRESSURE GAUGE HAD DROPPED. I CONFIRMED THIS WITH A FLASHLIGHT. THE NEEDLE WAS INDICATING BELOW THE GREEN ARC; YET NOT IN THE RED LINE. I FELT THIS COULD BE SIGNIFICANT AND THAT IT COULD HAVE BEEN THIS LOW FOR THE ENTIRE DURATION OF CRUISE; GREATER THAN 45 MINS. I REFERRED TO THE ABNORMAL CHKLIST WITHOUT AN ANSWER AS TO THE APPROPRIATE ACTION. I REDUCED PWR TO 50% WHILE I REFERRED TO THE ACFT MANUAL. I AGAIN FOUND NO ADVICE FOR THE SIT. I KNEW THAT BELOW THE GREEN LINE WAS UNDESIRABLE AND LESS PWR SHOULD BE USED. I TRIED SWITCHING INVERTERS WITH NO CHANGE. I TRIED REDUCING PWR ON THE GOOD ENG TO SEE HOW THAT OIL PRESSURE GAUGE REACTED. THE ONLY CORRECTIVE ACTION I COULD SEE WAS TO TURN AROUND BACK TO LAS AND LAND ASAP. I DECLARED AN EMER; ASKED FOR VECTORS DIRECT; AND PROCEEDED WITH THE PRECAUTIONARY ENG SHUTDOWN CHKLIST. I LANDED AT LAS WITH NO FURTHER INCIDENT; AND CALLED COMPANY MAINT. THIS WAS THE FIRST FLT OF THE NIGHT; LEG 1 OF 4. I WAS WELL RESTED AND WELL FED. I HAD BEEN FLYING THE BE1900 FOR THE PREVIOUS 2 WKS; AND THIS WAS A LAST MIN SWITCH TO THE BE99. I WAS RUSHED TO GET LOADED AND AIRBORNE; AND ALTHOUGH I SCANNED THE 'GAUGES IN THE GREEN' ON THE TKOF ROLL; I CANNOT CONFIRM THE EXACT READING OF THE OIL PRESSURE BEFORE TKOF. THE AIRPLANE I HAD JUST PICKED UP FROM AN OUTSIDE MAINT CONTRACTOR FOR AVIONICS WORK; I FELT THIS HAD NO BEARING ON THE SIT. I FELT IT WAS WISE TO SECURE THE ENG AND PREVENT POSSIBLE FURTHER DAMAGE AND LOSS OF REVENUE.