Garak - Yep, I would use an XP if the efficiency was up there, and did say that earlier. I've use regular WIX filters on the Vette a few times too, nothing wrong with efficiency of 95% @ 20 microns (Reg WIX/NAPA Gold) - anything at that point or better is good by me. Lots of filters these days are capable of 95% @ 20 microns or better, so after checking that box it boils down to construction, reliability (no failures), price and availability. I'm an Ultra fanboy, but not a total fanboy and have used many other filters over the years.