Your manual should tell you the oil capacity. Going by the dipstick is great if you "know" you are level. As far as oil goes I always ran straight 30 or 40 in my 454 and 350 mags. Newer engines might use variable weights now so check. I also have never run synthetics as I don't put enough hours on it per season and synthetics have a tendency to leak. If there is the tiniest of a leak, synthetic finds it. As far as longevity goes, I have yet to see a boat engine actually wear out, most blow up from something simple or stupid, not oil type. My cars I run Mobil 1, I have to actually, but it is more of a convenience to me. With newer cars, tolerances are EXTREMELY tight and synthetics are a must. Your engine is a 1960's engine made decades later, I doubt it matters. Then again if you run it hard, by all means do the most you can to extend its life. Then again for an extra $15-20 who really cares as long as it does not start leaking.
Nick
1994 22' Classic-454 B1 Red & white
1981 13' Whaler sport(original owner)
South Tampa Bay, FL "May I mamoo dogface to the banana patch?"