10-30 with a little zinc would be my choice, or 10-40 if its hot where you are. Synthetic will a little to slick for proper ring break-in, plus its fairly thin and tends to leak out more the conventional oil does. The oil filtering system, if it has one, is not as good as modern engines, so the time to change oil will still be fair quick so the advantages of synthetic oil will not be seen ( longer run life).
Joe
