I could never figure why these washers (for oil drain plugs) are called "crush washers". If you look at the washer that typically is used on a spark plug, it truly is a "crush washer" as it is made to physically distort and crush when the plug is first-time torqued into a head. However, all the washers I've ever seen used for oil drain plugs consist of a solid material that is soft enough to "mold" somewhat to the surfaces in order to seal slight imperfections and assist in holding the bolt/plug in place. They are either nylon, copper, or aluminum in construction, or else the plug head is lined with a hard rubber lip to provide this action. I haven't seen or used fiber as a washer material in this application, but it might also do the job. The best material I've used is copper, as it can remain fairly soft yet last repeated usage, where-as the other materials tend to score over time and start distorting too much to seal properly.
The other puzzle I have regarding these washers for oil plugs, is that they are usually recommended to be replaced each time the oil is changed? I can see this with the true "crush washer" construction as on spark plugs, as their crushing action is all used up with the first torque into a head - but who ever replaces those washers? Certainly nylon washers are the softest and often need replacing after a few uses as on a drain bolt, and aluminum also can get scored and need replacing, but the copper ones are a bit harder and seem to hold up indefinitely unless they are abused with a lot of grit or torque. Copper is a little too hard for some applications (i.e. on aluminum engine cases), so aluminum washers work best on aluminum engine cases, and will last a long time if everything is cleaned well before re-assembly.
