Date: 201808
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Takeoff / Launch
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We finished our checklists; verified the runway; and received our clearance from the Tower for takeoff. Engine acceleration was normal and the takeoff roll was uneventful. We performed a normal rotation and raised the gear. At approximately 1;000 feet AGL; we both noted the engine instruments had a white flashing rectangle around them and then saw a rapid decrease in oil quantity in the number two engine. We climbed to 2;500 feet; leveled off; and began to determine the malfunction with the QRH/QRG. We decided that we would notify ATC that we wished to return to the airport. The First Officer could not find a reference in the QRH/QRG that was similar to the issue we were experiencing. We elected to reduce the power on the number two engine to idle in case oil was actually being lost from the engine. Finding no relevant checklist; we elected to leave the power on the number two engine at idle; as the EGT was normal and surprisingly the oil quantity was slowly beginning to rise while we were on the downwind. The First Officer did a phenomenal job of talking to the company; briefing the flight attendants; working the QRH/QRG; and helping with the landing information while I hand flew the aircraft and talked with ATC in the pattern. We planned a flaps 15 landing; noted the aircraft weight and performed a single-engine landing with the number two engine at idle. The landing and roll out were uneventful. Leaving the runway; we had Airport Fire and Rescue follow us to the gate to look for abnormalities as well as shutting the number two engine down once clear of the runway. We taxied to the gate and notified all relevant company personal; OPS; dispatch; Maintenance; etc.; and discussed the situation/problem. We kept the passengers appraised of the situation as well and as how we planned to handle the maintenance issue/delay. We logged an ELB write-up and waited to speak with Contract Maintenance. While that was happening the First Officer and I discussed the event; debriefed what happened; and talked about what we could have done different/better. The First Officer made a very good point that he would have liked to stayed airborne longer and work the issue more before we returned and looking back; he was 100% correct. We worked well together to try and fix the issue and/or resolve the threat; but I placed too much work on him in too short of a period of time. We did both agree that the event was well handled and the outcome great. Both of us stepped back to the cabin after we conferred with OPS to deplane the passengers and then work with the flight attendants to debrief them about the event. Knowing what we could expect from this and in order to help alleviate the possibility of an adrenaline 'crash' both the First Officer and I each drank a large glass of orange juice. Soon after; I received a call from the [Chief Pilot] and we discussed the event with him and apprised him of the current situation. He asked us to perform the checklist for helping determine if we were fit to continue after this event. Contract Maintenance could find no oil loss or leak; added oil to the crankcase; check the cannon plugs and said we were fixed. Both the First Officer and I agreed to this but also inquired if we needed an engine run-up. It was determined that one was not required so we re-boarded the aircraft and departed again. The takeoff; cruise; and landing were uneventful with no abnormalities noted.


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


Synopsis: B737NG flight crew reported returning to departure airport after noting a decrease in oil quantity in the Number 2 engine.