If you end up rounding off the head there are a few options. They make special sockets that can grab onto a rounded off bolt. You can try to hammer on a slightly smaller socket. You could drill the head off of the bolt and use an extractor to pull the remnants out, but you risk breaking off the extractor and then you're in a real pickle. You could also weld a nut onto the bolt, but not recommended with a full oil pan. You can also take a hammer and chisel and try to get the bolt to turn using it at an angle, not recommended if you have an aluminum oil pan. You could also just buy an oil extractor and pull the oil through the dipstick tube.