I've been told: "if the particles reduce to graphite -ike dust when you remove them from the magnet with your fingers, its OK. If you can feel any particles then your motor may be doing some inappropriate internal self-machining".
Because of the way the dust "grows" from the magnet, it tends to look like alot of particles when you first take the drain plug out. Once you remove the clod with a thumb anf forefinger, it reduces to a very small amount of stuff.....that hopefully feels like a graphite emulsion and not sand!
Tom the petrochemical guru may weigh in with more.....