The N55 valve cover is made of plastic and houses the PCV (crankcase ventilation) valve internally. The plastic warps over time from heat cycling, and the integrated PCV diaphragm tears. Both cause oil leaks and vacuum leaks.
Symptoms
Oil smell from engine bay, visible oil seepage around valve cover, oil dripping onto exhaust manifold causing smoke, rough idle from PCV failure
Fix / Remedy
Replace entire valve cover assembly (cover + integrated PCV + gasket). Do not attempt to just replace the gasket ? the cover itself warps.