For really dusty conditions the larger syncro air filter works very
     well.

     The effectiveness of any air filter element is a product of two
     characteristics.  First the effective surface of the filter element
     and second the filtering properties of the element material.

     There was a study done by a heavy equipment mfg regarding oil capture
     mesh filters versus paper elements.  They found the paper element
     worked better than the K&N device for filtering properties.

     If you have a syncro you should also clean out the dust trap.  This is
     a little rectangular box upstream of the filter plenum.  It traps the
     larger dirt particles using a labyrinth swirl technique to cause the
     heavier particles to accumulate.

     The deadliest of all dust is glacier dust.  It's so fine that ordinary
     filters don't do too well at eliminating it and on the Alaska Highway
     and other inland graded routes 1000Mi oil changes and air filter
     changes are essential to forestall engine wear.

     al (US Army Arctic Test Center, Ft. Greely AK 1962-63)