Cetane & Lubricity additives whenever I fill the tank. When laying up I also top off my tank.
I also use the Sea Foam Treatment, but not for storage, i use it to clean my injectors once during the season. I will empty the filter, fill it with the Sea Foam, run the engine for a minute or so, enough to transfer the Sea Foam, then let it sit for 24 hours.
I use the Stabil "Marine" (it's green, not red) for lay up of all my gas engines.
Cheers, John