"Changed plugs" is this a petrol?
Whatever, cleaning the oil is not going to fix anything, you have to determine where it is coming from.
Worn piston rings, valve seals, or guides, turbo seals if it has a turbo, engine breather system, which points to worn rings.
Only one cylinder might be a leaking head gasket, or just the rings or valve seals worse on that one.
Looks like some dismantling and rectification work is needed.
If the engine is still in the vehicle, a compression check, wet and dry might help.
How did you find oil in the ports, what were you looking for, what were the symptoms/problem. We don't seem to be starting at the beginning here.