What was found
(CAN) OIL PRESSURE SLOWLY DECREASED DURING CRIUSE FLIGHT AT NIGHT. PILOTS SHUTDOWN AND SECURED ENGINE WHEN THE OIL PRESSURE FELL TO THE LIMIT FOR THE ENGINE OPERATING PARAMETERS. UPON INVESTIGATION THE AME DISCOVERED THAT THE OIL LOSS WAS FROM A CRACKED OIL FUEL HEATER AT THE BACK OF THE ENGINE. THE FUEL OIL HEATER WAS REPLACED, OIL ADDED TO THE ENGINE AND THE AIRCRAFT RETURNED TO SERVICE. WE HAVE HAD A SIMILAR FAILURE OF OVERHAULED FUEL OIL HEATERS TWICE IN THE PAST IN THE SAME MODEL OF AIRCRAFT.