I've had this discussion with a great mechanic with 40+ years experience. He feels that pressure and flow are too great inside the oil filter for a weak magnet to have full effect. I run a magnetic drain plug on all my cars. Inside the trans pan I have a strong "earth" magnet on my old HP cars (newer stuff already has a magnet in the pan), and on the outside of the oil pan on the engine. Oil will make its way down to the pan and get pulled to the magnet, even when the vehicle is not running.