Originally Posted By: LeakySeals
what is the efficiency of each?
M1 is 99% at 20 microns (which is the same rating as the Champ/Fram made Carquest Blues [if you can find them]), the Ultra is 99%+ @ 20 microns, also stated to be 50% @ 5 microns. The Ultra is almost always cheaper than the M1, and can be found on eBay for ~ $6 each when bought by case lots. The FPS (Fram Pro Synthetic) is the same as an Ultra, without the black grippy stuff-and some larger Ultras also don't have the Sure-Grip either. Really can't go wrong with either one. I'm almost exactly 1000 miles away from 20K on the XG3600 on the xB in my sig-I'll be posting the innards when I change it.