WD-40 is useless in this application. Get a proper penetrating oil such as Liquid Wrench or Kroil. Try several applications, waiting a half-hour or so each time to give it time to work. If it still won't budge, heat the bung red-hot with a propane torch to loosen its grip. You may still need to whack the wrench handle with a sledgehammer to break it free.
2011 335xi Individual 6MT - Azurite Black Metallic / Oyster & Black / Anthracite Maple + all the good stuff
N47° 35' 30.13" E11° 10' 33.36"
- End of break-in period. You can guess the rest. BMW CCA