With K&N's being alittle dirty does not hurt air flow through them, in fact it helps to filter the air better..
I have a carb, and a 10" x 3.75" tall K&N filter in a sealed cold air system, sticking up through my hood and into a cowl scoop..
Every 3,000 miles or so, I will take filter out, and tap it against my porch steps to knock all the little dead bugs, and bigger trash out of it, then reinstall.
Every 6K miles I give it a quick re oiling..
Then every 25K miles or so I will wash it with a mild soap and low pressure water, let it air dry, then give it a good re oiling