You will get a large difference in opinions on this one! Some will say that even though it's an older car you should still go for a modern synthetic oil. Others will say because it's an older car you should use a semi synthetic oil. Some will say you should use an oil that is 'thick' when cold (e.g. 10W/40 or 15W/40) to stop it leaking. Others will say that it is better to use a 'thinner' oil (e.g. 0W/40 or 5W/40) because it's kinder on the engine at startup...i.e. it's better that it leaks a bit than damage the engine with a thick oil (most wear taking place on start up when the oil is cold).

FWIW I use full synthetic 5W/40 (Fuchs Titan Supersyn) that was recommended to me by the chap at Opie oils.

Yet again tks Steve. I have some Helix 5/40 at home that I use to use on an F355. That Porsche recommended?

Don't worry about it too much. The main thing is to change it regularly eg. 5K miles or 10K if you are piling the miles on more quickly.
I've always used Castrol Magnatec 10w40 on classic Porsches and never had a problem (as recommended by Autofarm).