I modified my cars oil filer cap after this problem was reported on TM some time ago. On checking the seal it was found to be almost falling out so I drilled the centre of the cap and bolted a 40mm diameter washer inside to secure the seal and just leave the edge doing its job. It's a stiff nut inside, with Loctite and deformed threads, I don't want it to fall off inside the tank! I think this is similar to what other people have done but some photos explain it better. A cheap fix to a potentially expensive problem. Not sure how easy this is to do to the early tanks with a dipstick. Thanks andymot and others for drawing attention to this. Viewing TM for this sort of warning of problems is essential to M3W ownership IMHO.