B777-200 Captain reported loss of left engine oil quantity in flight; requiring an inflight shut down of the affected engine and a return to the departure airport.

Narrative

Take-off XA54Z Oil QTY was very full (28/28) before start. Oil QTY was 14 Left 16 Right at level off; FL400.Oil QTY Lo with back lighting showing 6; quickly dropped to 2 @ XC09 then 0 @ XC25. Between XC09 and XC25 the left throttle was brought to Idle and a turn coordinated back to ZZZ with a descent to FL260 (S/E alt on VNAV page). PIC SATCOM with MC/Dispatch. FO worked with ZZZZ Center. Direction was given by Maintenance to leave engine running until secondary indications. All steps of the low oil pressure procedure were completed except shutting down the left engine. Final RTB alt FL280 for military airspace. The engine remained at Idle until shutdown. No emergency declared at this time.Oil temp stayed steady ~100 and oil pressure fluctuated with the mean value decreasing. Wide abeam (21.3 miles) ZZZ1; the oil pressure was fluctuating around 74. [Requested priority handling] at that time. On base leg; outside ZZZZZ; for ILS XXR oil pressure went below min (70) triggering EICAS and the left engine was shutdown. ECL procedure was completed and Dispatch notified. Windmilling oil pressure was 9 on final. Alternate missed approach procedure was coordinated for single engine missed approach. Landing was uneventful. Landing was at XD36. Upon clearing runway; Tower asked if we were a PW; no response given.Post flight pilot inspection noted the oil cap was on. Oil was pooled under the engine and inside the bypass section appearing to originate from the front of the engine.