The most common failures are, white smoke coming out the exhaust and the engine running rough from the oil or the engine front will have a squeal coming out of the seal around the front of the crankshaft and this is usually is after the engine has warmed up. Also, you cannot easily remove the oil filler cap when the engine is running. A new AOS comes with the seal and boot for about $115. It takes about 2 hours. Failure of this part is common. I replaced mine and the top J-tube, left the bottom as is. Search here for AOS repair...there is one that goes step-by-step. also, here is Mike's DIY.
http://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages