The simple answer from Porsche is to use Mobile 1 0W40 sometimes labeled European and meeting the ACEA3 standard. A 2010 list of approved oils is at http://www.pedrosgarage.com/Site_3/Porsche-Approved_Oils.html
However, there are opinions by highly knowledgeable people who have studied used oil samples taken from Porsches for years who would suggest other brands and weights (depending on your climate) and who specifically strongly recommend against Mobil 1 0W40. Link to one such recomendation. Another recommends:
Castrol Syntek 5w40, 10w40
Total/ELF excellium/NF similar viscosities
Mobil 1 10w40 High Mileage
You can track this debate on http://www.bobistheoilguy.com/.
Porsche reviews "approvals" every 2 to 3 years. So an oil approved today would be reviewed, if requested by the manufacturer, in 2-3 years. And oils get reformulated so sometimes they lose their ratings. A peek into what the oil vendor submits to Porsche can be seen at http://www.lglubricantes.com.ar/productos/lubautos/cer/PorscheLetterJuly2009.pdf
There is a TSB 36/08 1701 from September 4, 2009 which really lists only the approved classes of oils and weights, not the specific brands and weights. It appears that recently Porsche has created a new Porsche specification A40 which required no retesting if you had a previously approved oil and was more an administrative change for the convenience of separating all sportscars from the Cayenne V-6 and any diesels.
My personal take...
- any 0 or 5 or 10 W40 synthetic oil meeting ACEA3 will be fine (depends on the temperatures in your driving neighborhood and if you have leaks (some of which can be ignored and dealt with by using the heavier oil but some are early warning signs and should be fixed..ask your mechanic).
- If you are in an always warm climate (40 F plus year round and no ski trips) then synthetic 10 or15 W40 meeting ACEA3 is OK. (especially if you are having oil leaks) though you may hear lifter noise on cold startup as the reason Porsche switched to 0W40 weight oils in 2002 was to cure this complaint. Earlier Porsches had come with heavier weight oils.
I have heard of people with oil leaks switching to the heavier weight oils (10W50, 15W50 but always ACEA3) and the oil leaks stopping.
I believe, based on input from mechanics with extensive knowledge of the internals of our M96 engine and whom I trust (and who have no chance of ever working on my car so they have no financial stake in the opinion) that a 5-6k oil change interval is far better for the engine than the factory recommended 12k (Porsche recently changed the recommended interval for normal service to 12k retroactive to 1984 models so your Owner's Manual is now probably wrong!). These mechanics further recommend an oil change every 6 months no matter the mileage interval with some going even farther and recommending every 3 months. This even for normal around town and in the country driving with lots of long drives that thoroughly heat up the oil. Most of us actually drive in a way that we should use the severe use oil change interval...even the factory recommends their interval be cut in half at least. (Severe because of the number of short stop and go drives and in traffic idling etc.) For racing the change must come after every race.