What was found
PILOT REPORTED VERY STRONG ODOR, CHEMICAL LIKE, PRESENT IN CABIN AND COCKPIT UPON BOARDING AND ALL OF FLIGHT. IRRITATING TO EYES, CAPTAIN, AND VERY BOTHERSOME. APU GENERATOR OIL DRAIN PLUG DRIPPING ON TO PLENUM. REPLACED SEAL WASHER, FOUND OIL LEAKING FROM GEARBOX AT COOLING FAN DUCT. INSIDE DUCT GOING TO OIL COOLER SOAKED IN OIL. COOLING FAN SOAKED IN OIL AND COKING APPEARS. REPLACED OIL COOLER FAN.