Since I own a Zetec I don't know the answer to this one: Does the Duratec 2.3L in the Focus ST use a standard spin-on filter or the cartridge system used in the 2.3L Fusion? If so, what years and what spin-on filter?
Assuming you are talking about the oil filter. Its a spin on type. To my knowlegde all Dtecs in the focus use the spin on type and you can get a lot of different ones. Amsoil, Fram, Bosch, Mobile 1, Wix, etc.