Best and Worst

The STP XL ones are pretty good, the standard STP is just a basic Premium Guard Chinese filter (or possibly an older Champ Ecore, like an ST).
Just an fyi. AZ STP XL are now Champ Labs US made, no longer Premium Guard made. Same goes for the basic blue STP, most newer ecore with metal endcaps. If interested, might as well go Super Tech MP series for the XL and ST series for basic blue STP. Save $4 and $3 respectively. My bet, same elements, just different can. Blue vs red silicone, MP series vs XL. Still showing previous PG made filters on website though, and efficiency info was for PG.

AZ STP come full circle, Champ made for a long time. Then went to off shored, now back to Champ.
 
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