Uh-oh. I just installed an Amsoil SDF-13 oil filter with Mobil 1 5w-30, and now it seems that the SDF-20 is being used on WRX's. The good people at SPMotorsports.com have never done me wrong, and they sold me the SDF-13 filters. Should I be worried? That filter is slightly smaller than the Subaru or Bosch filters I've been using, but seems to fit just fine. ????? ?????