I've been thinking about that test to see where the oil in the cylinders is coming from.
If you were to start your car and let it idle until it's fully warmed up and the fan kicks in a few times, the oil rings should expand and stop the leaking and the cylinders should clear themselves of any oil.
Then shut off your car and pull the plugs right away to verify that they are dry.
If they become wet by the next day after sitting, then the oil came from your valve seals.
Removing the plugs from a hot engine might be tricky though.