I decided to do some oil filter research after what we found while designing the Black Ops lifters. Considering a lifter does such a great job of trapping small particulate, I figured we should be running the best oil filters we could get our hands on.
Abrasive engine wear can be substantially reduced with an increase in filter single pass efficiency. Compared to a 40 micron filter, engine wear can be reduced by 50 percent with 30 micron filtration. Likewise, wear can be reduced by 70 percent with 15 micron filtration. That being said....
The following is a good rule of thumb. A filter is considered nominally efficient at a certain micron level if it can remove 50 percent of particles that size. In other words, a filter that will consistently remove 50% of particles 20 microns or larger is nominally efficient at 20 microns. A filter is considered to achieve absolute filtration efficiency at a certain micron level if it can remove 98.7% of particles that size. So, if a filter can remove 98.7% of particles 20 microns or larger, it achieves absolute efficiency at that micron level.
Most off-the-shelf filters are based upon a cellulose fiber filtration media. Most of these filters are, at best, nominally efficient at 15 to 20 microns. They won't generally achieve absolute efficiency until particle sizes reach 30 microns or higher. High efficiency oil filters have filtration media made of a combination of at least two of the following: glass, synthetic fibers and cellulose fibers. Those that use all three are generally the best in terms of filtration. Those that use only two will fall somewhere in between. The best of these high efficiency filters will achieve absolute efficiency down to about 10 microns and will be nominally efficient down to 5 microns or so.
I researched many different filter brands (of course some were private labels which crossed over) readily available for HDs and would you believe that the Harley Super Premium 5 brand was the only one I could find rated at a nominally efficient 5 microns?