Back in 2008 I posted a thread on here where I had let me Boxster go over 15k miles and 2 years between oil changes, and had an oil analysis done by Blackstone Labs. You can see the original post here ->
http://986forum.com/forums/showthread.php?t=18035
I had a goal of 10,000 miles for my next change, which I missed by around 1,500 miles and my actual interval ended up being 11,470 miles. Time between changes was less than a week shy of 2 years.
Here is my most recent Blackstone Labs report. The car has been running well although certainly starting to show it's age a bit as it comes up on 100,000 miles. I installed a new coolant reservoir last year about this time, and the air/oil separator just a couple of weeks ago. Still having a blast driving this car!