Didn't I just post this? From an engine professional:
Well, not quite, there is a simple difference with SI engines.
Sitting is better than idling - nothing good happens to an idling engine.
Each thermal cycle does some damage.
Moisture accumulates in engines and fuel tanks.
Here is the BIG difference**
If you are going to let the engine sit for more than a month, learn about "Fogging" the engine. This means spraying oil in it as you shut it down to prevent internal corrosion. (Diesels run so lean that this is not a big issue.)
Summary:
Take it out and run it hard every so often. Once a month is more
than enough. If you miss a month, don't worry.
Keep the fuel tanks full to minimize "breathing". Diesel fuel takes a very long time to get old.
Keep a charger on the battery(s).
I have had boats kept this way for YEARS.
Matt