CRJ200 FLT CREW HAS EICAS WARNING 'ENG OIL.' FLT CREW PERFORMS THE APPROPRIATE QRH PROCS AND DIVERTS TO THE MOST SUITABLE FIELD. THE TWR DECLARED AN EMER.

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UPON REACHING FL340 WE WERE IN PROCESS OF ACCELERATING TO CRUISE SPD AND COMPLETING THE CRUISE CHK WHEN THE TRIPLE CHIME 'ENG OIL' WITH L OIL PRESSURE WARNING MESSAGE ILLUMINATED. THE L ENG OIL PRESSURE WAS FLUCTUATING BTWN APPROX 23-28 PSI. FO COMPLETED THE QRH. THE L ENG THRUST WAS REDUCED TO IDLE PER QRH AND THE OIL PRESSURE RAISED TO 33-38 PSI AND OIL TEMP OF 112 DEGS. WE REQUESTED FL280 FROM ATC AND WERE CLRED TO FL280; WHICH ALLOWED A SLOW DSCNT NEEDED BECAUSE OF THE SINGLE ENG SITUATION. I WAS ABLE TO MAINTAIN NO MORE THAN 74% N1 AND STILL HAVE OIL PRESSURE 30-33 PSI. WE GOT ATC CLRNC DIRECT TO ZZZ AFTER TELLING ATC THAT WE HAD A L ENG LOW PRESSURE INDICATION AND HAD TO OPERATE AT REDUCED THRUST BUT BOTH ENGS WERE OPERATING. OUR APCH AND LNDG WAS NORMAL AND WE LANDED WITHIN 30 MINS. I ALSO BRIEFED THE FLT ATTENDANTS AND THE PAX AFTER OUR CLRNC WAS RECEIVED. DURING THE APCH BRIEFING; THE FO AND I DISCUSSED THAT IF WE HAD TO GO AROUND; WE WOULD PLAN A SINGLE ENG GAR SCENARIO; DUE TO THE REDUCED THRUST AVAILABLE ON THE L ENG.