15-50 is too thick for your car.......Porsche recommends Mobil 1 0-40.
Here is a good (but long) read on motor oil written by a very knowledgable source.
Read it and you will learn a thing or two about motor oil, what the numbers mean
and why you want to use 0-40 for your porsche
http://63.240.161.99/motoroil/