0W40 will be fine, change it once a year. It is easiest to just drive it every few weeks, when you get a nice day.
https://sites.google.com/site/mikefocke2/theboxsterinwinter
http://www.jmgporsche.co.uk/index.php/menu-articles/articles-maintenance/item/217-winter-storage-of-your-porsche
Excerpt from Mike's site, also reiterated in the other link:
"I change the oil in the spring because I don't accumulate enough miles on the car to call for a more frequent change. If I were storing it for longer or weren’t taking it out every few weeks, I’d change the oil just before I put it away for the winter under the theory that the old oil contains water and absorbed gases that can rot bearings. I'd also park it just after a long hot run that allowed the most burn off of those contaminants."
Porscha
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