Date: 201701
Local time of day: 0001-0600
Location: EGGX.ARTCC, FO
Flight conditions: VMC
Light: Night
Aircraft: B767 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center EGGX
Aircraft component: Powerplant Lubrication System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While enroute; the relief pilot and I were on the flight deck while the Captain was on her scheduled break. The relief pilot noticed that the left engine Oil quantity indicated that we were losing oil quantity. No visual or audible warnings were present during the loss of oil quantity. The left engine oil pressure remained at approximately 54-56 PSI. After a very short discussion about the situation; I rang for the captain to return to the flight deck. While she was returning I asked the other pilot to review Engine Shutdown and enroute diversion procedures with me. After doing so and monitoring the Oil pressure/quantity indications the captain returned to the flight-deck. We briefed the captain on the situation and had a discussion about the plan of action. We referenced the QRH and the MEL for guidance. The captain suggested we use the sat phone and brief the dispatchers and get Maintenance Control on the satellite line. We had a discussion with dispatch about our situation and then had a discussion with Maintenance Control. They gave us several scenarios including a failure of the left engine oil quantity sensor. The assumption was that it had failed because the left engine oil pressure remained fairly high and stable during this period of flight even though left engine quantity was reading at or near 0. The captain briefed the #1 Flight Attendant. It was my turn for a rest break; as there was no emergency yet and after consulting with the captain; I went to the back for my rest break. I was called up to the cockpit after a short rest. At this point it was clear the oil pressure was now dropping at a measured rate. We discussed our available enroute Alternates and plan of action. The decision was made by the captain to initiate an enroute diversion and advised ATC. We turned towards our alternate and descended 500 ft in altitude. Two other aircrews offered their assistance over 121.5. The captain briefed the flight attendants and made a PA to the passengers. We reviewed our memory items for in flight engine shutdown procedures and decided that we would initiate that procedure around 15 PSI on the left engine indictor as this was limit and also what Maintenance Control had recommended.Our diversion went well and we were transferred to Approach control. After accomplishing our checklists and briefing the arrival; terrain; weather and approach and flew the ILS without incident. Our lowest PSI on the left engine OIL indications was approximately 19 PSI. We did not shut down the left engine as it operated normally during the entire event. After landing we parked the aircraft on the ramp and waited for emergency crew and airport authorities to board the aircraft. After a short wait; ground handlers arrived and made a plan to deplane the passengers; accommodate them and then did the same for the crew. The crew then went to the hotel.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B767 flight crew reported a slow loss of oil from the left engine during an Atlantic crossing.