Burning oil after re-jetting 454

Oil change at the end of the season before parking the boat lets your engine sit in clean oil all winter. Sitting a long period with old oil may expose the bearings to whatever build up is in your oil at season end, water, Acid dirt. When I bought my first Nautique the Nautique dealer advised parking it with clean oil every year.   I do it most years.

Synthetics are great oils, no question.
For an old 351 engine that was designed to run on Petroleum based oils I stay with the good old Dino oil. (Dynasaur based Petroleum)
Synthetics have more heat protection and can lubricate better for a longer period of time in the right environment.
They are also thinner than petroleum based oils when hot so they can leak more than your standard dino oils.   When I say leak, that does not mean out of your engine onto the floor.
I am referring to how much leaks past the Main, Rod and Cam bearings.
Modern engines that require Synthetic are built to run synthetic oil so the Rod and Main clearances are tighter than what was used in the old days. With tighter clearances your synthetics perform to an optimum level.
WIth older engine loose clearances this can change.
When synthetics first started becoming popular in the late 80s many racers started losing engine bearings when they switched to synthetic oil. No other changes and the engines failed. Many of these race engines ran Main and Rod clearances at .0025 to .0032 and the engines failed.   The initial race talk was that Synthetic oil was bad and did not hold up to the demands of racing.
That talk all went away as they learned more about synthetics, new engines were assembled with tighter clearances and all the synthetic advantages were now positives but the new bearing clearances were held to about .002 Max for Rod and Main Bearings.
If I built a new engine I would use .002 max on rods and mains and then be comfortable running either type oil.   In an old engine with clearances unknown I would be more conservative and run Dino based Oil and change it more often. That is my 2 cents.
At the time these Race Shops were seeing engine bearing failures I was the engine bearing Rep calling on them, our bearings were blamed and it was front page news trying to get answers for these guys.   It was about a year before we had it figured out. Some of these engines had values up to $50,000 each so pressure was on to get answers they could trust.
FYI: There are race shops running 0/0 weight oil in drag engines because they figured out they dynoed with a few more horsepower, in a tight engine this worked.
This information is nearly 20 years old but the synthetic oil did not change its properties over that period. It is great oil and better but has some limitations.
Our old 351W engines were built fairly loose for Marine duty.
If your question was about an oil choice in a newer Nautique with a 5.3L or 6.0L I would still check with the Engine Provider to see if they modified the engines for Marine use with extra Main and Rod clearance. I would think they did not based on modern engine knowledge but I would certainly check. Boats get run much harder than Automotive engines so Oil is important.
I run the Castrol, Delo or Rotella 15-40W Diesel formula oil in my boat a 1995 351W.
It works with old flat tappet Non Roller engines just fine.
Use the same in my 1966 327/350 HP Chevy.