Rear Main Seal, due to overly loose tolerences on some m96/m97 engines. Usually just a mild oil leak, dealer typically replaces the seal up to three times, after that may replace the motor with a rebuilt unit if excessively out of tolerence. Occurs in 3-25% of 996/997/boxster engines. Very rare on gt1 style engines(GT2,GT3,TT), due to different block with 'tighter' tolerences. This has been a bone of contention for years with pcar owners and porsche.