This is from a FAQ on oils written by a Porsche specialist about the use of motorcycle oils. I've heard him speak and he knows his stuff. This would work for your low mileage and changing the oil at the end/beginning of the season.

Why use a motorcycle oil?
Motorcycle oils have higher levels of phosphorus/zinc for enhanced wear protection and the same high-temperature detergent technology for superior wear protection and engine cleanliness, even at elevated oil temperatures. Specifically motorcycle oils for aircooled engines are designed for very high localized oil temperatures and high overall oil temperatures, and typically have high flash points coupled with higher HTHS viscosities and lower noack% losses. As a whole, it would appear that all most motorcycle oils we tested have excellent anti-wear additive levels and most are not SM or SN rated oils, but rather earlier SG, SH, or SJ rated. In a pinch, it should be fairly easy to find a motorcycle oil with any of these SG, SH, or SJ ratings at your local auto parts store when it may be more difficult to get Brad Penn or Swepco, without having it shipped to you. Please do remember that motorcycle oils typically have levels of Zn and P that will kill catalytic converters, so if you have one, either remove it first or use another oil, like Brad Penn or Swepco. Also, motorcycle oils are not as detergent as the aforementioned Brad Penn or Swepco, so you must change the oil much more often, even though the perception of being able to go longer because the oil costs more is a false one.