What was found
DTW - FLT 423 - IN CLIMB CONFIGURATION, FLIGHT CREW OBSERVED NR 1 ENGINE OIL QUANTITY DECREASE FROM 16 QUARTS TO 6 QUARTS. RETURNED TO DTW WITHOUT INCIDENT. MAINTENANCE FOUND NR 1 ENGINE OIL TANK SERVICE CAP NOT SECURED. SERVICED ENGINE WITH OIL, SECURED OIL CAP. ENGINE RUN-UP CHECKED NORMAL IAW DC 9 MM, CHAPTER 79-11-0. ACCOMPLISHED AN OVERWEIGHT LANDING INSPECTION IAW DC 9 MM 5-51-0 AND NO DISCREPANCIES NOTED. AIRCRAFT RETURNED TO SERVICE. (X)