B777 Captain reported loss of oil in one engine during cruise. Flight diverted and safely landed with the affected engine still operating at reduced thrust.

Narrative

Flight was a maintenance ferry flight from ZZZZ1 to ZZZ. The aircraft was returning from a maintenance C check. The only maintenance item noted was a requirement to complete an ETOPS verification flight. All phases of flight went smoothly through climb out; at which point myself and FO (First Officer) Person A; the relief pilots; went on our approximately 5 hour break. Engine issues began while I was on break. At the point I returned to the flight deck; about 20 minutes before the conclusion of my break; the flight had diverted and established clearance to ZZZZ. The left engine was running; oil quantity was at 4 (right engine at 18) and oil pressure was approximately 40psi. I relieved Captain Person B after a briefing of current status so he could take a break. The pilot change brief included the previous discussions with Maintenance Control and the decision to divert to ZZZZ rather than ZZZZ2. Instructions from maintenance included continuing to run the engine up to full power if oil pressure remained above 30 psi; then reduce power to idle until 10psi; at which point the engine was to be shut down. Also; significant challenges were encountered previously communicating with ZZZZ Control. Mainly; no response to CPDLC messages; completely unusable static levels on HF; an inoperative regular SATCOM; and a 'number no longer in service' response on SATCOM. As a result; Captain Person B and FO Person C had requested priority handling; descended to 27;500 ft from FL380 and turned toward ZZZZ. They had just established communication with ZZZZ when I returned from my break; and were climbing to FL310. Soon after I began pilot flying duties; the oil quantity dropped to 1 and pressure steadily dropped to 30psi over the next half hour. I reduced power to idle. Coordination followed with ZZZZ Control to descend to block FL200-220; and the Driftdown QRC completed. Quantity then dropped to zero and pressure decreased and fluctuated from 19-25psi. In anticipation of shutting the engine down; FO Person A and I looked for an appropriate checklist. We brought up the ENG OIL PRESS L/R. The third step was to position the Fuel Control switch to Cutoff; so we held the checklist there until the agreed upon and soon anticipated 10psi or below. Meanwhile; Captain Person B returned to the flight deck and sat in the jumpseat. We discussed and made preparations for a flaps 20; single engine landing to ILS Runway XX to ZZZZ. Conditions were nighttime; clear; with strong winds at 220/26G40. Runway XYL was a longer runway; at 11;000ft; but with the strong crosswinds and a check of landing performance on Runway XX; at 8200ft; we elected to land on Runway XX. We switched seats back to Captain Person B with approximately 45 minutes to landing. Thorough briefings for approach and landing followed. Normal descent and landing checklist completed. Captain Person B noted an open EICAS; and after informing him we had completed the checklist to the point of shutting down the engine; no further discussion followed. This was well into the approach phase and attention returned to navigating successfully to a landing.However; the oil pressure never reached the 10 threshold. As such; we continued to run the engine at idle until after landing. Resultingly; the ENG OIL PRESS L/R checklist remained incomplete - inadvertently leaving the step for selecting GROUND PROX OVRD switch to OVRD unaccomplished. This significance of this became clear on final when the EGPWS alerted Too Low Flaps". The crew verified the intended flap position and Captain Person B verbalized his intent to continue. A smooth and safe landing followed.Fire and safety trucks followed us to the gate and noted no sparks or oil from the engine."