Have you checked vacuum at the oil cap. This could be caused by a failing AOS. According to Dr. Raby there are two failure modes for the AOS. It can fail in a condition that pulls oil from the oil pump pickup, sometimes accompanied by a high pitch squeal from main seal(s). Since max vacuum occurs at idle it's a possibility.
See discussion attached
http://986forum.com/forums/performance-technical-chat/21752-looks-like-aos-but-engine-woes.html