I changed my sons oil yesterday and discovered the drain plug head was partially rounded over from where the last OC place changed his oil. I tried using a 6 point socket, chisel and nothing worked. My last option was to weld a nut on the end of the DP and that did the trick. This was on his 2008 Ford Ranger and the DP was somewhat hard to get to. Here is a picture if anyone is interested. By the way the head on the DP was a 13 MM. I welded a 3/4 nut on to it, and replaced with a new DP.