If the washer came with the filter it goes on the filter canister bolt. If you remove the pan drain bolt you can reuse the old washer if you anneal it. Hang the washer on a piece of wire, heat it with a propane torch until it glows red and then quench it immediately. This softens the copper to the original hardness and you can reuse. Barring the high tech, most parts stores have an assortment of copper washers.