Wix recommends a 51085, but the bigger 51515 fits nicely too. There was a thread going on this topic that I revived, but I got told that's not cool. So, starting a new thread on the same topic. Does the larger 51515 equal greater filter efficiency? Obviously the new XP version of either choice is better than the non-XP, and I'm nearing an OCI so which filter is "better". Or should I avoid Wix altogether and buy a different brand?