2001 90hp Mercury outboard. This is the first plug replacement. My dealer sold me NGK's with a number of BUHW-2. Went to change them today and found the origional factory plugs were marked BUZHW-2. They are identical in apperance, the only differenence I see is the "Z". Anyone have any idea what the difference is? Is the "Z" something critical to my motor. Opinions please.