B777 FLT CREW REPORTS LOW OIL QUANTITY PASSING FL240 EASTBOUND OUT OF HNL AND ELECTS TO RETURN FOR MAINTENANCE.

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PREFLT; TAXI; AND TKOF WERE NORMAL. THERE WERE SEVERAL CELLS E AND S OF THE ARPT. AFTER TKOF; CAPT SUGGEST TO SELECT CONTINUOUS PWR FOR CLB OVER SOME BUILDING CLOUDS AND I AGREED. WE WERE GIVEN VECTORS AND FINALLY CLRED THE WX AND WERE PROCEEDING DIRECT TO CLUTS (OUR COAST OUT POINT). WE HAD ALREADY LOGGED ON AND I HAD ENGAGED THE AUTOPLT. THE CAPT WAS PRINTING OUT THE FLT UPDATE WHEN I NOTICED THE ENG PARAMETERS APPEARED ON THE UPPER EICAS. I WAITED FOR THE CAPT TO COMPLETE HIS TASKS AND I SELECTED THE ENG PAGE. I NOTICED THAT THE L ENG OIL QUANTITY WAS INDICATING ZERO. I BROUGHT THIS TO THE ATTN OF THE CAPT AND THE THIRD PLT. WE WERE CLBING OUT OF FL240 AND THE CAPT ASKED HCF FOR A LEVELOFF; WE WERE CLRED TO LEVEL OFF AT FL290. AS THE ACFT LEVELED OFF; I SUGGESTED WE SLOW UP A BIT TO REMAIN IN RADAR CONTACT. WE RECEIVED CLRNC TO SLOW DOWN. AS THE PWR DECREASED THE L ENG OIL QUANTITY ROSE TO APPROX 6 QUARTS. THE L ENG OIL TEMP WAS APPROX 40 DEGS WARMER THAN THE R ENG. THE OIL PRESSURE ON BOTH ENGS WAS NORMAL. THE CAPT CALLED DISPATCH AND GOT A PHONE PATCH TO MAINT. MAINT TOOK SOME SNAP SHOTS OF THE ENG PARAMETERS FROM OUR TKOF ROLL TO OUR CLBOUT AND NOTICED THAT THE OIL QUANTITY WAS NORMAL ON TKOF BUT BEGAN TO DECREASE SOMEWHAT ON CLBOUT. MAINT STATED THAT WITH THE ENG READING 1 QUART; WE STILL HAD 13 QUARTS IN THE ENG. MAINT SUGGESTED WE CONTINUE TO ZZZ AND MONITOR THE ENG PARAMETERS. THE CAPT AND I IMMEDIATELY LOOKED AT EACH OTHER AND DECIDED SIMULTANEOUSLY THAT WE WERE NOT COMFORTABLE FLYING THE ACFT AT NIGHT OVER THE PACIFIC WITH LITTLE TO NO OIL QUANTITY ON THE L ENG. THE CAPT TOLD MAINT THAT WE WERE GOING TO RETURN TO HNL AND ASK DISPATCH TO CHK OUR OVERWT LNDG PERFORMANCE TO SEE IF DUMPING FUEL WAS NECESSARY OR COULD WE LAND OVERWT. DURING THIS ENTIRE PROCESS; WE HAD A B767 CAPT THAT WAS ON VACATION RIDING IN COACH; TO LOOK OUT THE WINDOW AND SEE IF HE NOTICED ANY FLUID LEAKING FROM THE L ENG. HE CALLED BACK AND SAID HE COULD SEE A STREAM OF SOMETHING LEAKING FROM THE PYLON. WE REQUESTED A CLRNC TO RETURN TO HNL BUT DID NOT DECLARE AN EMER. WE ELECTED TO REDUCE THE THRUST ON THE L ENG TO MINIMIZE THE OIL LOSS (IF POSSIBLE). WE LEFT THE L ENG AT IDLE UNTIL TURNING FINAL AND THEN USED THE ENG AS NORMAL FOR OUR LNDG. THE RETURN TRIP BACK TO HNL; APCH; LNDG; AND TAXI BACK TO THE GATE WAS NORMAL. OIL LEAK DISCOVERED BY MAINT ON GND L ENG.