Yup, another common definition of a Boat. A hole in the water into which one pours money.
Seriously though. I deal with two stoke motors (opps! edit that to four stroke) that many have sat for decades in barns/garages.
And for those in more use, even with "normal use", they sit over the winter like boat engines.
I first learned about using Marvel Mystery Oil from mechanics I worked with in boat yards. It's the only thing that would keep valves from sticking in the old L-head Chrysler marine engines, found in so many cabin cruisers and boat yard slipway winch motors. And, it was not uncommon to find many marine inboard engines with a Marvel Mystery inverse oiler plumbed into the intake manifold.
I've used it in my antique auto restoration business for over 30 years. And, you'll see it used by other shops in a lot engines and fuel systems of very expensive antique auto engines.
It does wonders for keeping fuel systems free of rust and sludge. It also keeps fuel pump check valves from sticking when the alcohol gas dries out after a month or two.
And Marvel Mystery Oil has an affinity for carbon, so it's excellent at cleaning sticky carbon buildup off piston rings and valve stems when a motor is unused for months, and equally good at keeping them clean, when regularly added to the gasoline.
I buy it by the gallon for the shop for use in used and freshly rebuilt engines, and engines that are going into storage. Plus, every customer gets a quart can with a note for when and where to use it.
Also good for lubing and keeping air driven shop tools from gumming up.
Paul