How long is synthetic oil good for?

Change it once a year with normal oil. Fall, before putting it away for the winter would be the best time. You don’t even need to change the filter.

After a drive to get the stabilizer from the fuel tank to the motor (you do add stabilizer, correct?) drain the oil while it is warm and refill with regular 10W-30 oil. You’ll be ready to roll in the spring. Easy.