Date: 201810
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: IMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Powerplant Lubrication System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was the PNF (Pilot Not Flying) onboard Aircraft X from ZZZ. Weather was 040/09kts; 1 SM visibility; BR (mist); OVC002. We departed off RWY XX using Flaps 5 with Engine Anti-ice ON. Assigned to fly runway heading to 10;000 feet. Once level cleared direct to ZZZZZ intersection and climb and maintain 23;000. PF (Pilot Flying) commenced the turn and climb using VNAV and LNAV. Passing through approximately 18;000 the PNF noticed the left engine oil quantity at 6 quarts and decreasing. PNF remarked that he checked it prior to departure and knew that it was good. PNF mentioned to PF that we should level off and slow down to monitor the engine parameters. ATC was informed; 'we may have a potential problem and need to level off.' ATC granted an early level off and asked the nature of our problem. The PNF stated that the left engine oil quantity was decreasing but all other indications were normal. PNF and PF discussed the possibility that it could possibly be only the one gauge indicating a malfunction but decided that it would be prudent to turn back towards ZZZ in case other indications began to manifest. The PNF asked ATC for a return to ZZZ stating that the left engine was still operating but there was a possibility of an engine shutdown should the left engine indications continue to deteriorate. PF followed ATC instructions. PNF looked through 737 QRH for an ENG OIL QTY LOW checklist but there was not one. PNF did find an Engine LOW OIL PRESSURE and Engine High Oil Temperature checklist but neither of these applied. PNF began to review the Engine Failure or Shutdown checklist in case it became necessary to shut down the engine. Soon afterwards; the left engine oil quantity decreased to zero; the left engine oil pressure dropped to zero (?); and the left engine oil temperature turned red. The PNF then performed the Engine Failure or Shutdown checklist and secured the left engine. ATC was informed that the left engine was now secured; the need for delay vectors to complete all required checklists and program the FMC for the approach and landing; and the fuel remaining and number of souls onboard. The PNF sent a quick message to Dispatch stating;' Left ENG oil quantity zero. Return to ZZZ.' The PF flew all assigned headings and altitudes. The PNF completed the Engine Failure or Shutdown checklist and informed the lead Flight Attendant of the nature of the emergency and that we would be landing in approximately 15 minutes. The PNF then informed the passengers of the situation and that we would be landing shortly. The PF used the autopilot at all times and was able to program the FMC and set up the approach while the PNF was finishing up checklists; briefings; PA announcements. The PNF then ran the ONE ENGINE INOPERATIVE LANDING checklist using VREF ICE speeds. Landing weight was approximately 1;000 lbs below max landing weight; NON-NORMAL CONFIGURATION LANDING DISTANCE for One Engine Inoperative Landing Flaps 15 was adequate. PNF told the PF to continue to fly the ILS XYR approach to landing since he had a good feel for the aircraft and its trim condition. PNF quickly set up his own approach plates; and confirmed crossing altitudes; minimums; frequencies; etc. Both the PNF and the PF felt confident that the aircraft was well stabilized and ready for the approach and landing. PF flew the ILS XYR approach and landed very nicely. The ZZZ fire trucks had already been alerted and were standing by. Normal landing and taxi to the gate. Briefed maintenance crew of complete symptoms and the need to shut the engine down while airborne.


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


Synopsis: B737 flight crew reported that left engine oil quantity decreased to zero; resulting in engine shutdown.