RoyBoy
Diesel Dork
i think the best easy out there is are the Hanson - Irwin ones
IRWIN Hanson Hex Head Multi-Spline Screw Extractors - 522/532 Series
I agree, because they will slip in the drilled hole before they will break off, and usually they are successful. I also recommend the left hand drill bits
I will stand by drilling all the way through the bolt as well, its worth having to change the oil again (which I would assume the oil is not in yet being you were installing the cover when the bolt broke). I have extracted many, many bolts in my short lifetime