Is it spongy, like you could dent it with your fingernail? It might be chopped up sealant from an overzealous dealing job from a previous repair? Edit: Just saw consistency of hard rubber, so maybe?
Did you drop the oil pan cover and see, if it is this you may see some of this clogging the oil pickup screen.
Edit 2: Some possible causes here, (different than what i thought above)
My first REAL SCARE with the Boxster! need some advice..
Threaded Mode
