B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Landed As Precaution; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Inflight Shutdown; Flight Crew Diverted
Aircraft
While at en route cruise FL370 Flight Crew noticed a decrease in left engine oil quantity.  The white exceedance box illuminated around the secondary engine parameters and the left oil quantity soon went to reverse video.  At this time; oil pressure showed normal at 69%.  As we watched the oil quantity continue its slow drop; the decision was made to divert to [a suitable airport] out of precaution.  Knowing we were above MLGW (Maximum Landing Gear Weight); we requested to hold and burn down.  ATC directed us to hold as published.  I believe we were at 11;000 ft.  We lowered the gear; extended the speedbrake and dropped flaps to expedite the fuel burn.  Once the oil quantity dropped below 10% and anticipating a loss of oil pressure; [we requested special handling from] ATC.  Eventually with oil quantity reaching zero; oil pressure started to fluctuate between 37 and 53%.  At this point feeling the engine oil pump was beginning to cavitate; the Captain took controls and ordered the QRH ENG Low Oil Press checklist and subsequent Engine Shutdown Checklist based on the abnormal indications. We did not wait for the Low Oil Press light to illuminate because we knew it was soon to come. The left engine was secured.  Once MLGW was ensured; Captain landed [using] flaps 15.  Aircraft cleared the runway at the end where fire trucks were waiting.  Airport personnel inspected the left engine and gave the go-ahead for taxi to the gate. During this event; the passengers were well informed of the situation and the diversion.  Dispatch was notified and offered assistance as needed. Post flight inspection revealed the bolts holding the left engine Air Turbine Starter had sheared.  With the ATS (Air Turbine Starter) detached; engine oil which normally lubricates the seals was leaking from the breach.
B737-800 Captain reported diverting to an alternate airport after inflight shutdown following loss of left engine oil quantity.