Arguement #1 applies to the automotive field . Arguement #2 involves any engine with an air filter.
My specious arguement. #1 when a K&N oil impregnated air filter is used where there ia a Mass Air Flow meter is in the air inlet ( between the said filter and throttle plate) an accumulation of oil will get deposited on the two resistors in the MAF meter. This in turn causes the dust that passes the air filter to get stuck on the resistors in the meter. This accumulation acts as an insulator and changes the ability to read the airflow properly. When this takes place the MAF sensor cannot properly cleaned or serviced. It requires replacement. All General Motors Corp. vehicles will void the warranty if a K&N air filter is used.
#2 I do not know what exact micron size a K&N filter will capture, so I cannot argue that point. I do know I do not think much of washing a filter ( and allowing the trapped dirt to then get on both sides of the filter) and putting oil back on (not an exact science), it then putting it back in service.When I rode dirt bikes, I decided it was best to just buy a new air filter, properly oil it, and install it rather than taking a chance on washing the filter .
I want to state this is MY personal opinion. I am not on this site to argue with anyone. I have dumbasses on iatn.net to do that with.
p.s. there is nothing wrong with properly washing and re-using a $10.00 condom either.