So, I changed my oil today and noticed something strange when I pulled off the splash guard. The fram oil filter I used last change was a different part number than the one I had JUST bought. When I chaged my oil last fall, I used a fram PH3593A. The reference book that was at autozone didnt list the 04 wrx yet, so I used what it said to use on the 03...I thought -what could be different, its still an EJ20. So today when I went to autozone, the book was updated and it said that the 04 wrx 2.0ltr uses a fram PH6607. So i bought it and its a lot smaller than the 3539A. I double check the book and it lists the 2004 wrx as the 6607 but it lists the 2003 wrx as a 3539A. The updated book also said the 3593A was to be used on the 2.5RS engine. So my question is, if its an identical engine, why does it list two different filters for a model year difference. Will using the 3593A negatively affect my car in any way?? Also, according to the fram website, the 04 wrx uses the 3539A. HEEELP!