I and some other local Boxster/996 owners still use 15W-50 which was approved for many years, but no longer. I have a January 1997 and it work for me. I am in California and our winter is rain.
I have been Boxstering for almost 6 years. Porsche has changed the factory fill at least twice that I can remember, which means nothing to me but is important to some owners. Porsche sells the cars around the world. Porsche issues a new oil bulletin about once a year with a list of approved oils. Their bulletins do not match the weights in the owners manual. When Porsche issues a new bulletin Porsche does not say why there has been a change from the prior bulletin.
In fact, not too long ago there were serveral conventional mineral oils on the approved list. Now it is just synthetics.
I am not privy as to why to why Porsche switched to 0W-40. We guessed at one time the lighter weight oil was to reduce lifter noise at start-up, better fuel economy, or because it had a different approval.
I would keep your 0W-40 since that is what Porsche says at this point in time to use.
If this link works here is the most recent bulletin I have.
http://home.jps.net/~mjlopez/images/Box%20oil%20world%207-9-04.pdf
http://home.jps.net/~mjlopez/images/Box%20oil%20world%207-9-04.pdf
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