Oil leak confirmed. That makes a lot more sense than a fuel leak (wing/pylon/internal to nacelle) detected in cabin air as there is no path to bleed air in flight. Engine oil, on the other hand, is right in the air path to bleeds via the compressor stage and fan bearing seals.
Just speculating here - no inside information - but I'm thinking the fuel/oil heat exchanger failed in such a way that both fuel and oil were leaking overboard, but that fuel was also getting into the oil system - basically replacing the oil that was leaking out (the fuel is at a significantly higher pressure than the oil in the heat exchanger). That would explain the oil quantity not going to zero, and since Jet A doesn't lube as well as Mobil Jet it started causing bearing damage resulting in unusual vibes and oil/fuel fumes in the cabin.