fuel additives for winter storage?

You only store it for what? 5 months? That's what I do. You really don't need anything. Sta-bil used to tell you on their website that their product would last 2-4 weeks. I drop in a bottle of Seafoam, drive for a while, and park it. Seafoam is a gas stabilizer and a detergent, so it's good for the system at start up as well.