Change your oil
before placing it in storage so the aforementioned moisture and acids don't have the chance to work over your internals during the storage period. In the Spring, you're all set to go for the coming season.
All oil change intervals assume ideal conditions and very few people actually drive in these. Stop & Go, short hops, dusty conditions, wet conditions, prolonged idling, irregular fuel quality, hard driving, or any combination of the above all qualify as severe conditions and so the service interval s/b shortened.
Porsche doesn't provide a severe service interval, so you need to decide what interval you're most comfortable with. Using a Lab Analysis can help, but ultimately the decision is yours.
Personally, I drive my car in a similar manner to what you describe and I'm comfortable with an annual change just prior to storing the car.
