The second one seems the right size. But that head is big man. Make sure it doesn't stick out of the protective skid plate; you DO NOT want it hitting something on the road first.
By the way, the best way to loosen a stubborn bolt is with impact, like hitting the ratchet with your palm, rather than exerting ever increasing force. The 6mm allen bolt is plenty strong, but not if you tighten it to hell. Oh, and don't forget to coat the oil cap's gasket with a film of oil, so it doesn't bunch up when turned. I'm also doing all my maintenance; better pay for fluids and filters than letting dealer touch my car. Good luck, and please keep us posted if the magnetic plug's head doesn't protrude past the skid plate. Thanks.