I am / was the original owner of a 2004 Cayenne S.
I carefully maintained the car with oil changes every 7 to 10K miles, transmission fluid at 80K and the car was not driven hard.
Overall the Cayenne was great to drive but extremely expensive to maintain, numerous brake jobs and sets of tires.
The real disappointment was this past week after 9 years and 121K miles the engine failed.
The issue was a cylinder scoring leading to failure of cylinder one.
The Cayenne engine can not be rebuilt and a replacement engine is about $22K installed, used engines are about $8k installed.
Upon searching the web the engine failure is not unique and it seems like it is a major issue with old Cayennes.
Dramatically improve reliability, cost of maintenance and make an engine that lasts.