You don't want to be changing oil with the seasons, especially if your not putting that many hours on it. That gets expensive, is a PITA and is totally wasteful. Use a weight range that works summer and winter for you based on lowest and highest temps you'll "start" your tractor in. If your storing it in a somewhat or even unheated building the starting temps will be very different than the outside temps. Check it and see. I do agree that a 5W or 0W-30 will likely work best for you depending on conditions. Next would be a 5W-40.
Cheers,
DEWFPO