In a two cycle motor any oil is better than no oil.
Oil made for use in outboards mixed in correct ratio
of gas to oil is best. Manufactures of outboards do extensive testing under all conditions and make recommendations as to type of oil and gas oil ratio.
If oil is labeled TCW-3 certified it has been tested and found suitable for outboard use. Does maker of
your outboard recommend synthetic oil? Does the label on synthetic cil certify it as TCW-3?
There are many TCW-3 conventional oils on market and
several premium price synthetic oils being used in outboards with or without TCW-3 certification.
The questions are synthetic oils suitable and
are their claimed advantages worth paying for
are unanswered. In a new motor follow manufacture's
recommendations to maintain warranty. With older motors do what you think best.