What was found
AIR RETURN TO DEN, ON CLIMB-OUT, BOTH ENGINE OIL QUANTITY INDICATORS WENT TO ZERO. BOTH OIL PRESSURE INDICATORS WENT TO ZERO. BOTH LOW OIL PRESSURE LIGHTS ILLUMINATED. ENGINES OPERATED APPROXIMATELY 15 MINUTES IN THAT CONDITION UNTIL LANDING, WHICH WAS WITHOUT INCIDENT. BOTH ENGINE EGT'S, VIBRATION AND OIL TEMPS REMAINED NORMAL FOR DURATION OF ENGINE OPERATIONS. INSPECTED BOTH ENGINES AND DETERMINED OIL LOSS WAS DUE TO N2 GEARBOX ROTATION DRIVE ACCESS COVERS NOT INSTALLED ON EITHER ENGINE. BOTH ENGINES ARE IN THE PROCESS OF BEING REPLACED. ENGINE INFORMATION IS AS FOLLOWS: CFM56-3B2. NR 1 ENGINE, S/N 856183, 32111.70 HRS., 24091 CYC NR 2 ENGINE, S/N 857192, 32907.4 HRS., 24616 CYC.