The spark plug tube seals are leaking. You’ll need to remove the valve cover and replace the gasket as well as the tube seals inside of it. One thing to keep in mind, if the car was built in Mexico, the tube seals are not servicable and the entire valve cover will need to be replaced. I seem to remember Eric posting a video on replacing a valve cover gasket on an Accord. It’s not exactly the same as your Nissan, but the same principals apply. It’s a fairly common failure on those cars, especialy when they start to get old.
Yea it’s really not possible for the oil to come from inside the combustion chamber it would have to be an ‘external’ leak getting into the well, I would start with the valve cover seals as has been suggested.