Does this cilinder have compression? You can also have a weak spark. A weak spark is visible outside the powerhead but under load: pressure and gas mixture, it will not spark. You can try to switch the coils to exclude this. I am not familiar with this engine but I dont think the oiler can cause this problem of you have fuel in your cylinder. The seal leak I can imagine is a head gasket leak. Try to get an compression reader and measure the pressure on all cylinders.
