I agree on the NGK Plug. My father is a marine engine mechanic and he and others mehanics have told me that synthetic oil keeps your plugs from fouling faster but that if you use synthetic to stay with it because switching between nosythetic and synthetic can be hard on your engine because the synthetic will cause gaskets and other sealants that hold the engine together to wear faster. I have never tried it or tested it but that is what I am told.
Paul