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Dynaglo is intended for car use it has 8% E.D.L. (synthetic oil) and 2% castor oil, this is not enough oil for use with boats engines.
Duraglo is the mix you want for boat engines, it has 9% E.D.L. (synthetic oil) and 6% castor oil, with boat engines it is preferable to have more castor or the big end will soon break.
Synthetic only is not a good idea, even if you had say 15% you still need some castor to safeguard the big end.
Castor only at say 15% would be fine, you would certainly never damage your engine through lack of lubrication, but it is very messy, black gunge out of exhaust pipe and rapid carbon build up in tuned pipe.