AN MD80 PLT RPT OF A L ENG OIL LEAK WAS CHKED AND FOUND OK FOR SVC. ENG OIL/AIR SEPARATOR VAPOR COLLECTS ON ENG VENT COWL SEAL AND APPEARS AS A LEAK.

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DURING ROUTINE WALKAROUND; TECHNICIAN NOTED EVIDENCE OF OIL ON THE BOTTOM OF COWL. NOTHING TO MAKE ME THINK WE HAD ANYTHING OTHER THAN THE TYPICAL MD80 OIL ON THE LOWER COWL. THE CAPT SAID IT WAS NOTICED ON THE WALKAROUND AND HE WAS GOING TO WRITE IT UP. I TOLD HIM IT WAS 'TYPICAL.' HE STATED HE WAS OBLIGATED TO PUT IT IN THE BOOK. A QUICK CHK OF THE COCKPIT OIL QUANTITY INDICATORS SHOWED BOTH ENGS FULL AT 16 QUARTS. I WENT TO THE COMPUTER TERMINAL AND CHKED HISTORY FOR 100 DAYS. NO RPTS OF OIL LEAKING ON THE #1 ENG. I OPENED THE COWLING; VISUALLY INSPECTED THE COMPONENTS FOR OBVIOUS LEAKS; NONE NOTED; I ALSO CHKED THE CSD OIL QUANTITY SIGHT GAUGE AND IT INDICATED FULL. A PARTNER RAN THE ENG AT IDLE FOR SEVERAL MINS; NO LEAKAGE FOUND. THE ENG WAS SHUT DOWN; THE COWLING CLOSED AND WIPED OFF. THE LOGBOOK WAS SIGNED OFF BY MYSELF. THE MD80 HAS AN AIR-OIL SEPARATOR THAT EMITS AN OIL MIST OVERBOARD. THE MD80 ALSO HAS A VENT TO COWL DRAIN SEAL THAT PROVIDES A POOR SEAL; THUS VENTING MUCH OF THE OIL MIST INTO THE COWLING WHICH TO THE UNINITIATED COULD BE PERCEIVED AS AN OIL LEAK. OFTEN THE PLATE THAT MATCHES UP TO THE ENG VENT GETS DISTORTED AND FURTHER EXASPERATES THE POOR FIT OF THE COWL SEAL.