[felix83]

Not that I'm anything but a Porsche novice, but according to a, if not THE, leading Porsche mechanic in Sweden (Björn Waldegård's mechanic), the oil to use is Castrol GTX 15W40 mineral oil. Synthetic oil, especially modern stuff with many additives, can dissolve old gunk from inside the engine which may shorten its life span. Also, you don't want to use too light an oil, as it can exacerbate the oil leaks that old Porsches almost certainly have.

Also, don't buy your oil filter from Porsche. Apparently, they've changed their supplier from Mahle (good stuff!) to something else. The new filters seem to cause a larger degree of foaming in the oil tank. So make sure to buy Mahle or Bosch oil filters.

Again, I don't claim to know half as much as the other people here, but that's the information I've (recently) gathered from lots of places all round the 'web, from people who I know to be reliable and knowledgeable.

Good luck and welcome to the community!