NGK's are what is used from the factory and are recommended. Other spark plugs could cause issues.
I am guessing since your valve cover is starting to leak, it is probably also leaking around the spark plug wells. Oil is getting into the wells and fouling your plugs.
Replace your valve cover gasket and spark plug gaskets. Here is a write-up; http://www.2gnt.com/www/corbin/vc.html. After the new gaskets, put in a set of new NGK's properly gapped.