B767-300 Captain reported a loss of engine oil during cruise. Oil pressure remained normal; so the engine remained operational; and the flight diverted for an uneventful landing.

Narrative

I was assigned as the first officer on this fight and was the pilot monitoring. The following for sake of consistency was written from the Captain's point of view. At approximately XA25 and 2 hours of elapsed flight time I was preparing for my FAA mandated inflight rest period and the oceanic portion of the flight. As is my custom I noted on my personal copy of the flight plan current engine indications. It was at this time that I noticed the number one engine quantity was 6 quarts. To the best of my recollection it was 22 quarts at the gate prior to engine start; well above the minimum 15 quarts. At XA27 I recorded the following parameters for the left and right engine respectively. Oil Pressure 225/201; Oil Temperature 99/105; Oil Quantity 6/15; VIB 1.4/1.0. While it is normal for this Pratt & Whitney engine to show a large decrease in oil quantity after engine start and while running; this was atypically low. I discussed the situation with the two First Officers and told them to call me if the situation deteriorated. I left the cockpit and began my in flight break.During my break I remained awake and reviewed applicable sections of the Flight Manual as well as reviewed possible diversion airports that were not already listed on the flight plan. Approximately 45 minutes into my break I was notified by the purser that the FO's would like me to return to the cockpit. When I returned the left engine oil quantity was at 2 quarts and had been losing a quart about every 15 minutes as documented by the relief pilot. I returned to my duty station and they briefed me about items relevant to position; fuel status; communications; etc. We were in oceanic airspace and all appropriate procedures for oceanic entry had been completed. We had an ADS-C connection with ZZZZ Oceanic and a good SELCAL check with ZZZZ Radio.We then began discussing the oil quantity issue with the obvious first question to continue or turn back towards our initial ETOPS alternate. I knew from experience and FM (Flight Manual) review that Oil Pressure was the controlling parameter and not Oil Quantity. That said we all seemed to be appropriately concerned about the steady decrease in quantity over a relatively short timeframe. I divided duties within the cockpit transferring control and routine comms to the PM and ask the Relief Pilot to begin checking / verifying our ETOPS alternate was still viable (it was) as well as a potential nearest suitable if it became necessary. We continued in accordance with our clearance as I initiated a SAT call with dispatch and Maintenance Control and gave them the pertinent information about our situation.The initial part of the discussion with Maintenance Control revolved around the other engine parameters and a check of relevant history on this aircraft. I had previously flown the aircraft on or about several months earlier and knew that this was a new engine with under 100 hours on it. I verified with Maintenance Control that oil pressure was the controlling parameter and while unlikely this could be an indication problem. The maintenance controller did admit that 2 quarts of oil was a concerning indication. I paused the call and relayed the information from Maintenance Control to my crew and at that point I think we were all in agreement that while the engine was running normally and would likely continue to run for some undetermined amount of time even if the oil quantity went to zero; that a precautionary turn back was the correct decision given the information that we had at the time. It was at this time that I requested via CPDLC a revised clearance back to our oceanic entry point with an alternate airport to be communicated soon. This decision felt time critical in the moment since we were filed under 180 minute ETOPS rules and our CP (Critical Point) was rapidly approaching even though we had not been in oceanic airspace for very long. In hindsight and remembering that the fuel delta at the CP was almost 20.0 LBS it seems less critical now other than the obvious fact that the nearest land mass was behind us and we had already decided to turn back. This decision led to some communication issues that I will detail fully below. Suffice it to say that it is not possible in the 763 to conduct a SAT call and communicate over HF with ATC simultaneously.The discussion next turned to the dispatcher and what was available as an alternate. The dispatcher floated the idea of diverting to ZZZ for passenger accommodation and possible repairs with the obvious condition that the left engine continued to run normally at full rated thrust and no other signs of degradation. It seemed like a reasonable request and we discussed it as a crew. Our immediate concern was not only the weather conditions and the distance to ZZZ but having several available alternates along the way in case the situation deteriorated. The weather in the vicinity was challenging with various combinations of strong winds; low ceilings and visibility; and various runway and airport conditions related to freezing precipitation. After what seemed like an extensive discussion with dispatch and crew we decided on a stepping stone" approach of suitable alternates along the way to ZZZ. I made the decision to name ZZZ as the new destination and communicated that to ATC. It should be noted that at this time the engine was running and performing normally.After coordinating with ATC; the Flight Attendants; and the passengers; we settled in for the long divert to ZZZ. We continued to discuss weather and monitor our list of alternates. We reviewed the FOM diversion guide which we had used as a guide earlier but wanted to make sure we had covered all the big picture items. We also discussed how to evaluate the engine performance as we continued and focused on the oil pressure and temperature as the oil quantity was now reading zero. The engine continued to run normally.After approximately 3 hours and with about 1 hour remaining to ZZZ in the vicinity in ZZZ1 the Relief Pilot brought to our attention that the oil pressure on the left engine had begun to fluctuate slightly. It had been running steadily and approximately matched with the right engine but now it was fluctuating between about 165-170 PSI. More concerning was a discernible downward trend. When the pressure fluctuation reached 140 PSI we had a short discussion as a crew and decided while the pressure was still more than twice the minimum of 70 PSI it was probably wise to divert to ZZZ1 which was just left of the nose for approximately 80NM. We communicated our intentions and updated situation to ATC and requested priority handling as a precaution to give us flexibility to deal with any sudden change in circumstances.Somewhere during the divert I resumed PF duties and I briefed a two engine visual approach to RWY XXL at ZZZ1 with an ILS backup. ZZZ1 was VMC but had a quartering headwind gusting to nearly 40KTS. We also discussed contingency plans if the engine needed to be shutdown or just failed outright. Basically if we needed to secure the engine anywhere before the 500 foot call we would go around as necessary and complete all procedures IAW (In Accordance With) the QRH and FM. Below 500 feet we would continue to a landing unless we had a compelling safety reason to go around. We landed uneventfully and cleared the runway. The engine ran normally the entire flight and for nearly four hours with the oil quantity gage reading zero (but with otherwise normal indications except as detailed above). Once clear of the runway we shut down the left engine."