I use the STP variety on my Mazda Protege 5. I don't have many choices.
The Fram and Wix filters for this car are identical, the red fiber ones made in Korea. The STP, in my application, has more pleats, is white and fuzzy, and just "looks" like it would do a better job.
I prefer the AC-Delco filter for the GM V6's, but have had good sucess with the STP. They seem to fit the airbox bettter than some other after-market brands. Keep a reasoably clean one and you should be ok. If I couldn't get the AC-Delco, it would pobably be my next choice.
My Si numbers from uoa were acceptable with STP, but I'm hoping to lower them a bit with the Delco.
I think it depends on the application. I have an STP filter in my 4Runner and it's good quality and shows low silicon from the used oil analysis I've done.
I looked at one for my 5.0L Mustang and didn't like the overall construction so I have a Purolator in it right now...
Well today I to choose either Fram or STP for CA8039 (ford 4.6/5.4 trucks). Fram orange base had tons of flashing left over. Should have never left the factory this way if they had any idea what quality control means. It didnt have a code on the end of the media like the factory unit. I had a hockey puck end which would cause more turbulent flow. STP was a buck cheaper, no flashing, softer rubber gasket, was a bit of playing around with it to fit but seemed more equivalent to Motorcraft oem quality.
