What was found
ON APPROACH, DESCENDING THROUGH 9,000 FEET, THE LT ENGINE OIL LOW PRESSURE WARNING LIGHT ILLUMINATED. THE OIL PRESSURE MOMENTARILY FLUCTUATED BACK TO THE GREEN ARC, BUT THEN DROPPED DOWN TO ZERO PSI. THE CAPTAIN COMPLETED THE ENGINE OIL PRESSURE CHECKLIST, WHICH INCLUDES THE SECURING OF THE ENGINE, FOLLOWED BY THE SINGLE ENGINE CHECKLIST. AN EMERGENCY WAS DECLARED AND THE AIRCRAFT LANDED SAFELY. MAINTENANCE FOUND THE GEARBOX BOROSCOPE CRANK PAD COVER OFF. AFTER DOWN LOADING THE DIGITAL FLIGHT DATA RECORDER, MAINTENANCE LEARNED THE LEFT ENGINE RAN FOR 2.5 MINUTES WITH NO OIL PRESSURE. MAINTENANCE REMOVED AND REPLACED THE LT ENGINE. AIRCRAFT RETURNED TO SERVICE.