I use a marine synthetic oil in my 3M20 A, & have never had a problem. I started using it after my initial break-in period, so for those with older engines this is not an option. In addition, I don't use additives in either oil or fuel. I have in the past but, not for years now.
I also use synthetic oil in my car & motorcycle with great results & less internal wear.
When I do an oil change, I run my engine 30 minutes & the hot oil pumps out easily. I check the old oil for particles from oil breakdown of metal wear but, I find none.
As for quality fuel, I jerry can it to my boat. Never from marina tanks. I use one of the major brand gas company diesel pumps. I've posted this before but again, the fuel I get is almost aviation quality. It is the color of water, it's really filtered. I don't have problems with filter clogging & tank sediment like I used to. I would not use fuel that has any brown tint to it, as it is not filtered enough.
The best way to ensure good fuel is to USE IT, not let it sit for months or more.
Some may disagree with me on this however, this method works for my friends & I.
CR