A DC9-30 WAS DISPATCHED IN NON COMPLIANCE WITH #2 ENG OIL TANK FILLER CAP MISSING RESULTING IN LOSS OF OIL.

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ON JUN/XA/00 ACR FLT WXX ARRIVED IN ZZZ AT XA35 FROM XXX. I WAS TO CHK THE ENG OILS AND MY CO-WORKER WAS TO PERFORM A WALKAROUND CHK. I SVCED #1 ENG ADDING 2 QUARTS AND CLOSED THE TANK CAP AND OIL SVC DOOR. I THEN PROCEEDED TO #2 ENG AND ALSO ADDED 2 QUARTS. AT THE SAME TIME MY CO-WORKER OPENED THE CSD SVCING DOOR TO CHK THE OIL LEVEL. I SAW THAT HE LEFT THE CSD SVCING DOOR OPEN AND WALKED AWAY; WHICH TO ME MEANT THAT THE CSD NEEDED SVCING AND HE WAS GETTING THE SVC CART AND OVERFILL BOTTLE. I FEEL I CLOSED THE OIL TANK CAP AND SVCING DOOR AND THEN POSITIONED THE STAND TO SVC THE CSD. WE SVCED THE CSD; CLOSED THE DOOR. WE THEN WENT TO THE COCKPIT. I ENTERED THE OIL ADDED IN THE LOGBOOK. THE ACFT DEPARTED AS FLT YXY TO WWW AT XB55. ACCORDING TO THE LOGBOOK ENTRY BY THE FLC AFTER TKOF ROLL; #2 OIL QUANTITY DROPPED TO 6.5 QUARTS. THE ACFT DID NOT RETURN TO ZZZ BUT DEVIATED TO YYY WHERE ACCORDING TO THE MECH SIGNOFF IN THE LOGBOOK; FOUND #2 ENG OIL CAP OFF. OIL SVCED; ENG RAN; NO LEAKS. I FEEL TO HELP LESSEN THE CHANCE OF THIS OCCURRING; THAT IF THE OIL TANK CAPS WERE CHAINED TO THE OVERFILL LIP OF THE TANK; YOU COULD LET THE CAP HANG OUTSIDE THE SVCING DOOR AND THAT WAY YOU WOULD HAVE TO REINSTALL THE CAP SO YOU COULD CLOSE THE OIL SVC DOOR. ALSO; IF IT WAS CHAINED YOU COULD NOT LEAVE THE CAP ON A WORK STAND.