is there an ASTM efficiency rating listed anywhere for the K&N? Can't find....
I don't think ASTM has a current efficiency spec. ISO 4548-12 is what's been used for automotive oil filters for the last 20 years.
List of test standards used in industry: http://www.pecofacet.com/Portals/10/Documents/lab-test-standards.pdf"ISO 4548-12 - Methods of tests for full-flow lubricating oil filter for internal combustion engines - Part 12: Filtration efficiency using particle counting, and contaminant retention capacity."
Here's all that K&N says on thier website in the FAQ section:
https://www.knfilters.com/oil-filters- WHAT IS THE EFFICIENCY RATING OF A K&NÂ® OIL FILTER?
Each K&NÂ® oil filter is designed to meet or exceed OEM specifications for efficiency, and each design is laboratory tested for efficiency, capacity, and burst strength.
You'd have to contact K&N Tech line to see if they will give you efficiency numbers. And they may just tell you "It's propitiatory information", which is a cop out IMO. Just what is the "OEM specifications for efficiency" anyway? ... lol. Nobody really knows.