Narrative:

Preflight; taxi; and takeoff were uneventful; and followed SOP (standard operating procedures). Approximately 1 hour into cruise; and under radar control with center; we passed over a waypoint and noted our time and fuel remaining for the fpr [flight plan route]. In addition; we selected the full engine display and immediately noted the left engine oil quantity indicated 10 quarts. The right engine indicated 18 (normal). The captain and I concurred that 10 was not normal. Within 5 minutes; it indicated 9. The captain sent an ACARS message to our dispatcher. I suggested a sat phone call to [operations control] which included tech/maintenance; which was done. We all concurred that if the left engine oil quantity dropped to 8; we would return to ZZZ. Within minutes; the oil quantity indicated 8. The captain passed control of the aircraft to me and began coordinating our return; calling our [relief pilot] off break; advising our flight attendants; and informing the passengers. I contacted center; requested and received immediate clearance to ZZZ. With the captain concurrence; I reduced the left engine to idle; and descended to FL210; selecting 270 knots (avoided the need for max N1 on the right engine). The captain continued to coordinate with dispatch (desk 81) requesting arff as a precaution; as well as the required information for hazmat loaded in the forward cargo. The captain resumed his role as pilot flying for the remainder of the flight. We [requested priority handling] with approach control; and made an uneventful (overweight) approach and landing on runway xxr. Our debrief was simple: it was a textbook R9; with a few lessons learned along the way.apparent oil leak on a high-pressure oil line (per maintenance).

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Original NASA ASRS Text

Title: Flight Crew encountered engine oil leak and landed uneventfully.

Narrative: Preflight; taxi; and takeoff were uneventful; and followed SOP (Standard Operating Procedures). Approximately 1 hour into cruise; and under radar control with Center; we passed over a waypoint and noted our time and fuel remaining for the FPR [Flight Plan Route]. In addition; we selected the full engine display and immediately noted the left engine oil quantity indicated 10 quarts. The right engine indicated 18 (normal). The Captain and I concurred that 10 was not normal. Within 5 minutes; it indicated 9. The Captain sent an ACARS message to our Dispatcher. I suggested a SAT phone call to [Operations Control] which included Tech/Maintenance; which was done. We all concurred that if the left engine oil quantity dropped to 8; we would return to ZZZ. Within minutes; the oil quantity indicated 8. The Captain passed control of the aircraft to me and began coordinating our return; calling our [Relief Pilot] off break; advising our flight attendants; and informing the passengers. I contacted Center; requested and received immediate clearance to ZZZ. With the Captain concurrence; I reduced the left engine to idle; and descended to FL210; selecting 270 knots (avoided the need for max N1 on the right engine). The Captain continued to coordinate with Dispatch (desk 81) requesting ARFF as a precaution; as well as the required information for HAZMAT loaded in the forward cargo. The Captain resumed his role as Pilot Flying for the remainder of the flight. We [requested priority handling] with Approach Control; and made an uneventful (overweight) approach and landing on Runway XXR. Our debrief was simple: It was a textbook R9; with a few lessons learned along the way.Apparent oil leak on a high-pressure oil line (per Maintenance).

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.