I try to never reuse nylon washers if it can be helped.The heat they are exposed to makes them brittle over time.Then they crack,and you've got a leak.But the copper plugs can be reused many times without worry as long as every thing is clean when you reinstall the drain plug.Just check them for cracks from overtightening,or pitting from dirt.You don't allways get a new one with the filter.I don't know if they forget to put it in the box,or if simply fell out someplace.If you don't get a new one,and the old one looks good,don't worry about reusing it.