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I would believe that the Amsoil test is very accurate. They probably hired an outside company to do the actual testing. There seems to be a big difference between the Honda & Toyota filters and the others. This to me says one cannot just assume, "Oh its an OEM filter so it must be a good filter". As a side questions I wonder what kind of specs Honda and Toyota have for these filters. Maybe the spec just states some sizes (diameter, thread) and bypass pressure, and does not get into efficiency.
Without trying to question the test, I think it's only natural to look at it critically without knowing who did the test, ie. was it an independent lab? Not doubting the Amsoil filter results as this is the published efficiency they advertise. No doubt it's a great filter, and I'm not saying otherwise here.
The other filter results are likely accurate too, giving the iso test# and it is relatively recent. That plus the fact that three of the other OEM's showed comparatively good results to the Amsoil filter.
As for the OEM's giving spec's regarding efficiency, generally that information is proprietary, not published. Hard to find information regarding efficiency of OEM's. Based on these results though it seems the no end cap design isn't very efficient, as said, just as r_r's tests also showed here on Bitog.
And I am curious whether the Fram Honda A-02 is the same as the Tough Guard as thought, and that the TG's99% @ >20um means much greater than twenty microns? Or, the Honda A-02 media is very different and just not very efficient? Does give me some satisfaction for using a P1 than either Honda oem.
OTOH, the three other oems showed decent results.