Oil on the spark plug 1996 Mazda Mx-5 Miata?
A small bit of oil, if on the firing end or at the initial
threads is acceptable, but may indicate wear along the rings or
head gasket. Oil in the area above the crush washer and threads
(where the plug wire is connected) could indicate that the plug was
not in tightly enough, or that your valve cover gasket needs to be
replaced.