1. case: dusty roads in the country => You require a CAF with heavy dust loading capacity = triple layer accumulation media with medium efficiency, something like MERV 7 should be ok.
2. case: city roads with heavy fine dust concentration + VOC => You need a HEPA-filter e.g. MERV 16 + activated carbon.
But if You use such a filter in the above case, it will clog within hours. And its cost is relevantly higher.
3. case: heavily humid areas (south-east Asia etc.): You need something that achieves coalescence and draining (membranes/surface treatment/silica etc.).
So, basically OEM filters will necessarily be of the first type, in order to be "universally useable" and achieve long lifecycle and low cost (for them).
As always, it's left up to the end user to evaluate his specific situation and go for (aftermarket) alternatives if necessary.
Last edited by michelino1968; 04-16-2012 at 04:22 AM..