Date: 201502
Local time of day: 1201-1800
Location: ZZZZ.Airport, FO
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
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 Diverted; Flight Crew Landed As Precaution; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We had a normal departure. After crossing into [oceanic] airspace we noticed the #1 engine was reading 40% which got our attention. As we continued to watch the oil qty decrease we determined our best course of action was to divert to the closest station. Congestion on the frequency necessitated our use of the call pan-pan to break in. We declared an emergency and requested clearance which was immediately received. We were a little more than an hour out. We got an HF phone patch to dispatch / [Maintenance Control] and notified them of our intentions. The oil qty indication continued to decrease and before the descent indicated as low as 21%. Once the nose was pointed down there was a slight increase in the indication. Our arrival was normal / uneventful. At the gate with the engine shut down for a period of time the oil quantity read 40%. Contract Maintenance arrived and conferred with [Maintenance Control]. The residual oil was cleaned from the engine and cowl and the engine refilled with oil. We ( the crew ) preformed an idle engine run at the gate while maintenance observed the engine. It was determined that the oil was coming from the IDG drain pad tube but not the location of the actual leak. The rate was measured and [Maintenance Control] determined that it was within acceptable (at idle) parameters. They said we were good to go. My personal decision was that to me that it was not acceptable and I would not take the aircraft in that condition. A leak at idle is one thing it's an entirely different matter at high power and without a real fix I would not take that aircraft over the water (class 2 ) at night to an intended destination with poor weather. The oil quantities were looked at on the ground but it was the situation where I can't recall the actual numbers but know they were acceptable. The oil had been topped off prior to arriving but I have no idea when the leak developed and had nothing to gauge the loss against.  [Flight was cancelled]. We had the passengers on the aircraft the whole time prior to the cancellation due to the customs issue and the hope that the leak might be a quick fix.


Second narrative: Upon maintenance inspection; the oil leak was isolated to the IDG. In coordination with the [duty manager]; the crew determined that further flight on the engine without proper maintenance was unsafe; so the remainder of the flight segment was cancelled.


Synopsis: B737 flight crew declared an emergency and diverted to nearest suitable after they noticed loss of oil quantity in the right engine. After landing the crew refused to fly the aircraft on to the final destination when Maintenance determined the problem could be deferred.