Here are a couple of threads where we discuss different options for sealing the oil drain plug. I still use the PTFE orings with no issue (not a gasser, but same dimension plug). As GMP says in one of the threads, the oring crushes down easier and provides a better seal than the copper or steel washers.
http://www.gasgasrider.org/forum/sho...highlight=ptfe
http://www.gasgasrider.org/forum/sho...highlight=ptfe