You boat looks to be 6 or 7 years old. Boats 25 years old have similar engines that run well, as does your starboard engine.
For mine it's a question of time and money
. If the money is no problem, and you're short of time have the engine pulled, sell it on Whatever and have a new one installed. End of problem.
If you have plenty of time and no money I'd suggest assuming that the bores had glazed and act accordingly.
There have been suggestions that changing to a straight mineral oil
and running the engine hard is the way to go. Other on this Forum will know way more than me.
The worst course of action in terms of both time and money is to chase unknown faults at umpteen dollars an hour and expensive parts
with a mechanic
. I'd suggest do it yourself or don't do it. It might be expensive lessons but by the time you've got the problem sorted you could be as good as some mechanics. A nice way to go cruising.
There is, after all, the starboard engine if the port goes kaput.
Do keep in mind that my advice is worth exactly what you're paying for it.