I can't think of any reason not to use synthetic other than the cost and
this cost is of no consequence in the normal boaters budget. The loads these
automotive engines are running under in boat duty is much greater than in
the usual car or truck, so use the best oil you can find, i.e. name brand
synthetic. That said, most any good oil will do fine as they have in the
past, but why not use the best. That's my two cents.
Spark plugs? use what they recomend.

--
Ron White
My boatbuilding website is:
www.concentric.net/~knotreel