Good question, I used to own a 150 hp HPDI, and they recommended running it in the outboard.
In Europe Yamaha Ring Free was/is not availabe and i never used it on my 2001 twin HDPI till i blow one in 2005.
It seems that one piston ring got stuck by carbon build-up and get catched by the intake or exhaust window at 000 rpm and BANG!
The engine was completely trashed and the piston was grinded to aluminum powder .... Total loss ...
The reason for this "accident" was probably a combination of a very windy season leading to long hours driving slowly, not using Ring Free and using cheap TCW-3 oil
Using Yamaha Ring Free seems to be mandatory for Yamaha 2 stroke engines and other brand 2 stroke engines will probably benefit as well.
It is also a good habit to get the best, ashless / low ash 2 stroke oil to reduce carbon buildup.
I never use any additives in any of my cars and boats even when i store them each 6 months for another 6 months.
I use a fuel system cleaner when i dewinterize the older engines but i feel that i just waste money.