A DC9-50 WAS DISPATCHED IN NON COMPLIANCE WITH THE R ENG OIL FILLER CAP NOT SECURED. INCURRED MAJOR LOSS OF ENG OIL. ENG NOT SHUT DOWN.

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A DC9 ACFT ARRIVED AT MY GATE IN ZZZ AT THE BEGINNING OF MY WORKDAY (2ND SHIFT) WITH APPROX A 1 HR GND TIME. I DID A VISUAL WALKAROUND INSPECTION; THEN ENTERED THE COCKPIT TO CHK THE GAUGES FOR OIL LEVELS AND TO LOOK AT THE LOGBOOKS. BOTH ENGS WERE LOW ON OIL; AND THE L ENG WAS ACTUALLY WRITTEN UP. I ASKED A CREW MATE; 2 GATES OVER; TO SVC ONE OF THE ENGS WITH ME. I PULLED THE TUG WITH OIL BOWSER UP TO ENG #1; AND MY PARTNER SVCED IT. I ASSISTED HIM WHILE HE COMPLETED THE SVC AND CLOSED IT UP. I MOVED THE TUG TO ENG #2 AS HE WALKED OVER AND WAITED. HE THEN RESET THE QUANTITY INDICATOR; TURNED ON THE ELECTRIC PUMP; AND HANDED ME THE SVC HOSE. AT THE SAME TIME; I OPENED THE SVC DOOR; REMOVED THE OIL CAP; WIPED IT CLEAN; AND INSPECTED THE O-RING FOR DAMAGE. I THEN BEGAN ADDING OIL TO THE TANK. DURING THIS WHOLE TIME; IT STARTED TO RAIN; WHICH WAS A DISTR; AS WE DIDN'T HAVE ANY RAIN GEAR ON. THE PARTNER; I ASKED TO HELP; (A MANDATE WHEN DOUBLE OIL SVCING A DC9) WAS THE NEWEST MEMBER ADDED TO OUR CREW; AND I HADN'T WORKED WITH HIM BEFORE. ALSO; THE FLT WAS DUE TO DEPART SHORTLY; AND LOGBOOK ENTRIES STILL NEEDED TO BE MADE. AS WE WRAPPED UP AND STOWED THE SVC HOSE; WE AGREED TO MEET IN THE COCKPIT. HE WALKED AND I JUMPED ON THE TUG AND DROVE IT AROUND TO THE JETWAY STAIRS; NEGLECTING TO SECURE THE OIL CAP. INFLT; THE CAPT NOTICED A CONSIDERABLE OIL LOSS AND CALLED IT IN TO MAINT CTL; BUT DID NOT HAVE AN INFLT SHUTDOWN. THERE'S NO EXCUSE FOR NEGLECTING TO COMPLETE A TASK; BUT THERE ARE CONTRIBUTING HUMAN FACTORS. FOREMOST IS THAT I GOT OUT OF MY ROUTINE. NORMALLY ON A DOUBLE OIL SVC; 1 GUY COMPLETELY SVCS 1 ENG AND I SVC MY ENG BY MYSELF; FROM THE FIRST STEP TO THE LAST STEP; BY MY HABITUAL ROUTINE; THINKING ABOUT EACH NEXT STEP. I THINK OUTSIDE INFLUENCES; LIKE TIME; WX; AND OTHERS CAN'T BE ALLOWED TO AFFECT THE THOROUGH COMPLETION OF SAFETY RELATED TASKS.