The spark plug gasket is leaking. Clean the spark plug boot with degreaser or a strong detergent because the oil will deteriorate the boot material. I would then go down to Nissan or a good auto parts store and order a new valve cover and spark plug gaskets along with a tube of RTV silicone for the corners and around the rubber plugs at the end. Then reinstall the valve cover tightening no. 1-5-6-4 in that sequence to 3 ft-lb or 4 N.m then in the numerical sequence fom the center out to 7 ft-lb or 9.5 N.m of torque.
Tightening sequence
F 7-----5------2-----10
r l --4----1----------11
o 9-----3------6-----8
n
t
Troy
Tightening sequence
F 7-----5------2-----10
r l --4----1----------11
o 9-----3------6-----8
n
t
Troy
