K&N reusable air filters are definitely worth the extra cost upfront, as they are designed to last the life of your vehicle. Fitting a K&N high flow air filter is the simplest, most cost-effective and least technical way to increase engine performance. K&N filters are reusable, and come with a 10 year or 1 million mile warranty. The standard air filter is almost never a restriction point.
I've been using K&N filters on my cars for around 10 years and I'm still not sure if they're really worth it, but I've never had any issues with my mass airflow sensor (MAF). I should disagree with this general statement. In a normal passenger car? Sure, it's probably not worth it. However, once you find the need for a high-performance filter, the K&N is an excellent choice.
I bought a K&N outlet years ago for my Subaru, and that same filter is still in use and looks new when clean. CRC Sensorklean can be used to remove oil from MAF. A couple of original equipment manufacturers have it listed as a first step before replacing an iirc mass airflow sensor. I know Chrysler does it for sure.
If you grease the filter too much, then yes, it will damage your MAF sensor and even clog your throttle. A simple solution for that would be to simply not overgrease the filter. How do you know if you're getting too fat? In addition to videos you can find online and tutorials, the cleaning kit describes how to properly lubricate the filter. Find an offer (eBay, Amazon) to drop in and try not to pay full price. K & N itself is decent.
But it's almost impossible to clean them well, oil CAN affect the MAF and dirt sticks to the oil, meaning filters can clog more quickly. If it's for a daily driver, I'd say no. If you're trying to modify a project car, there's no reason not to, but I highly recommend Specter over K & N. You'll never get a big power boost with a cold air intake, but it doesn't sound so bad. It depends on what your goal is.
If it's to increase HP, no, it's not worth the money. If you're looking to eliminate the need to buy an OEM-style replacement air filter every time you make your adjustments, it will take you a few years to get the most out of it. But if you're looking to increase MPG, an air filter that provides increased airflow to your intake manifold while keeping the air relatively clean and free of debris will give you an increase in miles per gallon. If you've ever been to an auto parts store, you've probably seen air cleaner boxes that claim extra power and more torque than the factory filter. But being so cheap and easy to install, it's hard to believe that these filters actually produce observable gains. Well, Jason Fenske from Engineering Explained put these claims to the test, and it turns out that yes, high-performance air filters actually produce more power. Not all K&N are complete replacement boxes; you can get replacement panel filters to reduce noise and still get an upgrade.
But the previous point is good; if your engine is not the one at the top of the range but still uses the same intake configuration, you will probably see little gain from it. It's a case-by-case scenario, and asking in a bicycle forum will leave you with a bunch of people who believe or don't believe in them based on their use in their car rather than theirs. My advice would be to decide why you want it. It's not just for full power (restrictions affect pumping losses at all revs, least of all down), but it probably won't make much of a difference unless your intake is badly designed or you've modified the rest of the engine to suit it (improved EV etc.). It means you have a filter that you can clean and re-grease which is good but some find that they contaminate airflow sensors if a little excess oil is applied so that may be a consideration. An air filter helps seat the air more evenly instead of the air hitting the AF sensor in large chunks which obviously makes the fuel supply more uniform. Using a K&N Power clean is a better option as an alternative; buying the refill and service kit that includes a cleaner and specialized K&N air filter oil. And the third common problem hot air pollution: The warm air from the engine is ingested through the air filter. I put in a cold air intake and eventually my car would shut down at the most inconvenient times (i.e., roundabouts) and it ended up being the mass airflow sensor that failed.
I change my air filter approximately every 20,000 miles and at $10 each time I would have to keep my car for 100,000 miles to make the K&N worth it. The trademarked red filter (usually conical in shape) on display or its kit may include a closed air box. He finds that on the test bench aftermarket filters actually produce more than OEM units with largest gains produced by K&N filters (around four horsepower and five pounds). It suggests that increase in power could be result of less filtering especially in case of cheaper CarQuest filter although because he had no way of testing filtering process that aspect was not observed. What Fenske doesn't prove is how well each aftermarket filter actually filters air entering engine compared to original unit. Warm air is not as oxygen-dense as cold air and as you may recall air-fuel ratio is very important when it comes to horse making. While replacing only OEM air filter with K&N high flow filter will add power and increase acceleration biggest gains come from installing complete cold air intake system. I remember reading article in Performance Ford...