WD-40, CRC 'Lectra-Clean, Liquid Wrench, and Kroil
I've always had reasonable success heating the part, then as it cools applying PB Blaster. Time. Then applying mechanical impact -- that seems to be the trifecta.
A few taps with a hammer, or an impact wrench, or the back of a crescent wrench, almost without fail will loosen the suspect nut or bolt.
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