Carefully file the rounded edges square as much as possible. Select a
socket that seems slightly too small but sort of wants to go on. Gently
tap that socket on with a hammer. Then try to loosen the bolt being
sure to keep the drive square and holding it tight to the bolt. If the
bolt is metric, which it probably is, then the tightest fitting socket
may be an SAE size.
PS> Have a replacement bolt available and perhaps an oversize one for
if the threads strip and even a backup universal rubber plug if nothing
else seals it. And don't let the wife change oil next time.
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