The myths swing both ways on K&N.
It's the facts of K&N that are more relevant.
K&N uses 2 more layers of cotton gauze for their diesel truck filters than they do for passenger cars.
For off-road and dirty areas, K&N recommends the use of a foam pre-filter.
K&N themselves after Arlen Spicer's visit to K&N admitted outright that their filter was a compromise between flow and filtration. (But don't admit that in advertising)
K&N also admitted that their efficiency figures are based on testing using constant and not variable dust loading and admitted their filters don't fare as well during a variable loading test. (But don't admit that in advertising)
As I have said many times before, K&N air filters are a valid product with a valid application. K&Ns only crimes are misrepresentation by omission, and recommending products for inappropriate applications.
I guess the "admitted outright that their filter was a compromise between flow and filtration" is a duh to me - of course! The issue to me w/r to K&N or any other high-flow filters is this...does you car need an extra X% of filtering efficiency? Does that X% make any meaningful difference? In the K&N testing (Spicer) that is so famous on the interwebs, the difference is about 3% if I am remembering correctly (96% for K&N and 99% for all the paper ones with AC Delco being highest?). And that graph was quite misleading as th axis started at 90% (again, from memory here) so tha the K&N looked way worse than the others. Last UOA I did on my Golf running a K&N had the comment "excellent filtration" based on low Si and other numbers - not everyone has issues with these. K&N produces an after-market performance product and like all of these companies, markets it accordingly.
Percentages are also misleading, especially in a narrow range.
A percentage point can represent multiplication. In the case of K&N, if we forget the percentage point completely, we find that it allowed 18 times more dirt than the AC Delco filter.
There, the "percentage system" was far far far more charitable to K&N than the actual dirt passage figure revealed. This is why I don't understand people getting mad over the percentage figure. That figure is doing K&N the most favors, making the dirt passage sound small by comparison.
I'd rather hear "3% worse" than "18 times worse" any day of the week if I were them.
And let's keep in mind that this is their 7-layer media that let 18 times more dirt through. How much worse do you think the media is with only 5 layers?
All this is going on is a very gentle test. In the real world, things get quite nastier. This is how I have ended up seeing so many K&N filters in front of some very dirty intake tracts.
I understand that Blackstone told you "excellent filtration", but Blackstone will say that for any engine with low Si numbers. They've said that for marine UOAs I have sent in. Do boats have "excellent filtration"? No. They don't. I can put a full strip of paper right through a marine flame arrestor. If I poured sand on the flame arrestor of some of them, I bet I could get every grain through without making a mess before the engine blew up. Metal slats or tremendously spaced screens.
It's not "excellent filtration" it is "favorable conditions". Your Golf is an on-road vehicle that likely does not exist in a harsh environment. The truth of the matter is your vehicle is likely benefitting from both filtration and favorable conditions.
Is it really obvious that a performance filter is a compromise? Most K&N buyers barely know what an air filter is, and K&N isn't telling them any better, so how would it be obvious?
Does a vehicle need "X filtration"? Some do. K&N will literally sell you one for anything at all. The point is that K&N could care less what your filtration needs are or are not. They're going to sell you one anyway.
Coming back to the metal flame arrestors, I actually replaced all of mine with K&N filters. Why? Any cotton gauze beats that junk, and K&N media is a USCG certified flame arrestor. Perfect application for K&N.
K&N does NOT market their filters accordingly to the fact that their filters are a compromise. That information is NOWHERE in their advertising. They even used to market fuel economy benefits, which is complete horse manure, and no longer do.
They are an entire species of gorillas when it comes to marketing, but it is not at all accurate.