B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance
Flight Crew Diverted; Flight Crew Landed As Precaution
Aircraft
Approximately 30 minutes after reaching cruising altitude; the lower display illuminated indicating low oil quantity in #2 engine. It indicated 3. Oil pressure and oil temp matched #1 engine. We opened QRH and learned there is no NNC (Non-Normal Checklist)  that covers low oil quantity. Upon a deeper review of our manuals we learned via the 737 FCTM (Flight Crew Training Manual) that the controlling engine indications that could lead to an inflight engine shutdown in regards to engine oil are only pressure and temp. We decided that as long as pressure and temp were good we would continue normal operations. Also based on aircraft logbook and Local Maintenance briefing regarding an earlier flight with a low oil quantity indication we felt it was an indication issue.We monitored pressure and temp and also made contingency plans for an engine shut down just in case. Within 15 minutes the quantity indication dropped to zero; but temp and pressure still matched the #1 engine.  After another 15 to 20 minutes we noticed oil pressure was slowly declining with  temp still good. Oil pressure continued to decrease until illuminated red. Ran QRH LOW Pressure NNC; which directed us to the inflight engine shutdown NNC.We informed ATC that we were performing a precautionary engine shutdown and will need to divert to ZZZ. ZZZ was chosen based on our cruise altitude and distance at providing the best drift down and descent airport. We completed the inflight engine shutdown NNCWe then completed engine out landing NNC and proceeded to ZZZ and landed without further incident. Upon landing we stopped on runway and were towed to ramp.Low oil quantity.
737-80 flight crew reported loss of engine oil inflight.