Oil that appears on spark plug, in a typical 4 cylinder engine; such as car or lawn mower, can indicate that a problem exists with the valve trane or piston rings. If the piston rings are involved this commonly referred to as blow by. In the case of a mixed style, 2 cycle engine, that can indicate that the oil mixture is too strong or incorrect for the motor type. In either case, check your manufacturers instructions for cleaning or replacing spark plugs. Also, verify that you are using the correct oil mixture.