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.
r l --4----1----------11
to add to what KA24Tech just said - did you check the spark plug to see if it was loose? when i first bought my altima 3 years ago, for the first three days, i couldnt figure out why the spark plug wires wouldnt stay on. i went to buy new plugs and found out that the spark plug wire in question was connected (sometimes anyways, lol) to a very loose spark plug. the compression was pushing by the plug and unseating the wire. tightened it down and the problem was gone...