I believe the 50/50 acetone/ATF works... that said, how many of us use heat if the fastener won't move with reasonable torque? Hitting acetone + ATF with a torch would be "interesting".
It's not that dramatic, it does burn and I'm sure some hamfisted mekanik has burned some car/building/themselves/other. The idea is to gradually build heat, then spray the penetrant. Going back and forth on torque helps too.
