Have you stopped to consider that the fouling of the plug might be fuel, not
oil? I see a lot of talk about
oil...not much (if any) talk about fuel.
A high resolution picture would be real helpful.
My first guess would be carbon fouling due to misfire. A bad plug wire can do this. Once the plug becomes carbon fouled, it no longer fires reliably which leads to more carbon fouling...it cascades until you have a soot covered plug.
It could be as simple as wires, cap, boots, the plug itself, etc.