Oil Drain Plug Washer Size

[jtopiso_imp]

In case you need one in an emergency, you can use one off a spark plug, such as a common NGK "B" series plug. Better than using a flattened-out washer that's curling up over the lip.

good idea.

good idea.
Another optiion is to re-anneal a used copper washer.

Just let it hang from a wire, heat it with a torch unti it's red, then let it cool at ambient temperature. It will be soft again, and will crush & seal on the next tightening.