The oil is mixed with a lot of coolant (the coolant tank you see is only the coolant overflow tank, so it shows only a small fraction of the overall coolant in the system), so the oil in the tank you see will not be all black like oil itself. Instead, it would look like what you're looking at now...a frothy tan colored milkshake-like mix. Hate to say it, but it's almost certainly an oil/coolant intermix, and you can't keep driving the car without addressing it because you will really damage the engine.
If you can't afford to fix the car at a dealer or independent shop right now, buy a Bentley Repair manual and you can perhaps try to replace the oil cooler/heat exchanger yourself to see if that was the cause of your intermix. Of course, you'll also have to flush and refill/burp your coolant system tank as well. If that doesn't do it, you'll really need to take it to shop that knows Porsches.
Repair manual:
http://www.amazon.com/gp/aw/d.html/ref=redir_mdp_mobile/185-9993856-2479431?redirect=true&tag=porschepetesboxs&a=0837613337
2001 Porsche Boxster
2007 Carrera S Cabriolet
Threaded Mode
