As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) ON WALK AROUND, MAINTENANCE DISCOVERED OIL ON THE COWLING. FURTHER INVESTIGATION TRACED THE LEAK TO THE OIL COOLER BYPASS VALVE. UPON REMOVAL, THE BYPASS VALVE O-RING WAS FOUND FLATTENED AROUND THE COMPLETE CIRCUMFERENCE. IN ADDITION, THE BYPASS VALVE SHOWED CHAFING BETWEEN THE SLEEVE AND VALVE. IT ALSO SHOWED CHAFING ON THE INNER SHAFT. THE BYPASS VALVE WAS REPLACED AND THE AIRCRAFT WAS SUBSEQUENTLY RELEASED FOR SERVICE.