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Like Bill said... full tank for storage. If not full I would make it totally empty.
I think that fuel without ethanol will last in storage a surprisingly long time as long as the tank is full. I wouldn't bother with adding a stabilizer just for one winter season with a full tank that does not have ethanol.
As far as using a premium grade gasoline it is not necessary. The higher octane premium gasoline is only needed with a high compression engine to prevent pre-ignition (knocking). Your A does not need it nor does it probably like it.