What was found
NR 3 ENG OIL PRESSURE DROPPED TO 15 PSI AND LIGHT ILLUMINATED THEN WENT OUT WITH OIL QTY DECREASING TO 8 QTS INITIALLY THEN CONTINUED TO 6 QTS. INFLIGHT ENG SHUT DOWN ACCOMPLISHED IAW CHECKLIST. WINDMILL TIME 25 MINUTES W/ POSITIVE OIL PRESSURE. PER PILOT REPORT SHUT DOWN ENG AS PRECAUTION. EMERGENCY DECLARED. APPROACH AND LANDING UNEVENTFUL. PERFORMED ENGINE TROUBLESHOOTING ON NR 3 ENGINE REF DC-8 MM 71-00-00-1 FIG 141. FOUND O-RING ON OIL FILLER CAP DAMAGED. SERVICED ENGINE WITH 8 QTS OF ENG OIL. REMOVED AND REPLACED OIL FILLER CAP. RAN ENGINE FOR 5 MINUTES. FOUND NO OIL LEAKS, ALL INDICATIONS NORMAL.