[GoingTooFast]

I'm sorry for the off topic but for those who are not so familiar with these engines you can see that the only way for the oil to get into the spark plug threads (above the tips) is through the cylinder head cover gasket (#6 in the picture):





This is what you see when the cylinder head cover is removed (pictures are from someone else's engine):








From the above pictures you can see both the spark plug and injector wells side by side in the middle of the head (between intake and exhaust valve springs), also where the cylinder head cover gasket goes and how oil can get into those wells if the gasket doesn't seal 100%. I didn't find any cut view of the N54 engine so the following picture of the N55 engine also illustrates where the spark plugs and injectors are mounted in the engine. You can clearly see where the cylinder head cover meets the cylinder head around the spark plug and injector wells area which has to be sealed by the gasket to prevent any oil from the valve train to get inside those wells and mess up the spark plugs: