No more replies necessary. The last gentleman that took the time to respond to you is a true EXPERT with marine engines. He has tore down and rebuilt them along lake Ontario for over 30 years. When he talks engines everyone I know listens.
It really comes down to whether or not you have a catalytic converter, and how you use the motor. The advice about zinc is sound science, and for many years SAE 30 weight has been the standard in small block marine engines, unless it is "tired", and then some bump it up to SAE 40 when the air temps warm a bit.
I've owned several sets of 5.7's/350s and the best advice I can give is watch/tune your carbs so they don't "dump" gas into your oil, and be a guage watcher and don't let them get hot--replace impellers as necessary.
We are all battling the fraud of ethanol gas, so make sure there are water separators in your fuel lines.