AC68 PILOT REPORTS OIL PRESSURE LOSS AND SUBSEQUENT ENGINE FAILURE RESULTING IN AN EMERGENCY DIVERSION.

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ON PREVIOUS LEG IT WAS OBSERVED THAT R ENG WAS LEAKING OIL. FOR HOMEBOUND LEG; OIL WAS TOPPED OFF. WE WERE MONITORING PERFORMANCE CAREFULLY ON DEP FROM ZZZ AND DURING INITIAL CLB. PRIOR TO CALLING APCH TO REQUEST FLT FOLLOWING WE NOTICED DROP IN OIL PRESSURE ON L ENG. THIS SITUATION DISTR US FROM MONITORING OUR ALT RELATIVE TO CLASS B AIRSPACE. WHEN WE MADE THE CALL TO APCH WE HAD ALREADY CLBED THROUGH THE 4000 FT FLOOR OF THE CLASS B. APCH ADVISED US OF THE SITUATION AND REQUESTED WE DSND FROM 5200-4500 FT; WHICH WE DID IMMEDIATELY. UPON REACHING 4500 FT (LESS THAN 1 MIN) WE PASSED INTO THE NEXT CLASS B RING WITH A FLOOR OF 6000 FT. THE L ENG CONTINUED TO LOSE OIL PRESSURE IN THE CLB TO 11500 FT. TEMPS STAYED IN THE GREEN. SEVERAL MINS AFTER LEVELING OFF WE OBSERVED OIL STREAMING FROM THE L ENG. WE NOTIFIED APCH OF OUR INTENTION TO LAND IMMEDIATELY AT ZZZ1. ON THE DSCNT INTO ZZZ; THE L ENG FAILED. IT WAS FEATHERED AND SECURED. BECAUSE OF UNKNOWN CONDITION OF R ENG; WE DECLARED AN EMER AND LANDED WITHOUT FURTHER COMPLICATIONS. THE R ENG CONTINUED TO OPERATE NORMALLY UNTIL SHUTDOWN.