I just went through this same dilemma. While changing my oil last month I discovered that the magnetic plug on the gearbox did not include a crush washer. I was sure that I had forgotten it at the last oil change. I rooted through my parts and found a bag with one crush washers in it that was labeled "Magnetic Plug" and it fit, so I installed it. While at Oshkosh this year, I visited the Lockwood Aviation booth and asked for a supply of new ones. They searched their inventory, searched the parts diagrams and came up with nothing. Finally, they called and spoke with their Rotax technical department at Sebring and were assured that there is no crush washer intended for that plug. Long story to a short question, but the answer from the professionals is that:
"No Crush Washer Is Intended on The Magnetic Plug"
By the way, the oil tank drain plug crush washer is a pretty good fit, and I can't see any harm in adding it. ..............Tom
2013- RV12, Kit #119. N123M First flight Nov21. It's a keeper!
1998- RV-9 tail kit, built and sold
1989- RV-6 tail kit, built and sold