When in doubt, always use the oil weight recommended by the factory. It’s usually fine to switch to synthetic oil if your vehicle calls for conventional (but not the other way around). Synthetic oils generally last much longer than conventional oils.

I would affirm the tugboat captain’s experience. Over the years we owned two 19′ powerboats with I/O engine/drive setups. I used synthetic engine oils exclusively for these engines. The engine internals stayed spotlessly clean, and I got the synthetic’s benefit of good lubrication during that first cold start in the spring and every subsequent start after each boat sat on its trailer for a spell. More importantly, marine engines are always working under a load; no coasting down hills ever. Synthetics add engine longevity under such heavy duty conditions.
Interestingly, the second of the two boats had a Volvo-Penta outdrive. The manufacturer specifically recommended synthetic gear lube for that relatively complex gear drive system.

Contrary to conventional oil, a full synthetic oil breaks down much more slowly and has consistent lubrication properties throughout its service life. It is also less prone to sludge buildup and more resistant to heat. In a turbocharged application, your car needs this heat resistance to prolong the life of the turbo components. It’s best to follow the manufacturer’s recommendations if the manual calls for synthetic oil.

When it comes to oil, it’s usually best to stick to the manufacturer’s recommendations. If my vehicle required full synthetic and someone used a synthetic blend instead, I’d change it just to be on the safe side. If you can show they made a mistake, you may be able to get them to put the correct stuff in at no charge to you.

Is that all true?

In my opinion, I’d highly recommend to continue using conventional oil. Older cars were simply not manufactured with the tight tolerances of modern cars. Synthetic oils have different flow characteristics vs conventional and will find tiny cracks and crevices which can result in leaks.