Two stroke oil is a good idea, but... any water that condenses inside the tank will sink under the oil at the bottom (oil floats on water) and cause rusting there. Filled with gas and a little bottle of gas line antifreeze (to dissolve any water that condenses) is best for a bike you will ride the next season. I kept a few tanks in an unheated shed for years, a friend suggested putting a handful of mothballs in an old sock, tying a bit of string to it, then stuffing it in the filler with the string out... then closing the cap on the string (so it could be pulled out later). I did this. The tank on a CB77 I had parked in there with no gas but otherwise untreated rusted badly. The ones with the mothballs stayed shiny inside.