Any penetrating oil (liquid wrench, ect.) will work. On tough bolts USE A GOOD QUALITY SIX POINT (Rather than 12 pt) SOCKET, like Sears or Allen, ect., not a cheap import. The reason is, 12 pt sockets have a very small gripping are, and so the amount of pressure/sq in can be 3-4 times as much on the flats of the bolt heads as with a 6pt socket which uses the full face of each side of the hex to spread pressure. Good quality (but more expensive) sockets have tighter tolerances so they fit better, slip less and round out fewer bolt faces.
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