This is an OEM Subaru 2.5L oil pan, required to fit some aftermarket Headers on a 2002-2005 WRX. The 2.5L oil pan allows for clearance of the crossover pipe on some aftermarket headers. This is the stock oil pan on all 2.5L turbo Subaru models. The item description for each header will tell you if you need to purchase this item.
Subaru STI 2004-2007
Subaru WRX 2002-2007