Through years of working on automobiles of all kinds, I tried ATF fluid as penetrating oil, especially on rusted nuts/bolts, and found it to work reasonably well, but I never did any comparisons or tests on results. W. Ellis Brown, who wrote the two editions of Building Double Rifles on shotgun actions told us, at Tech. College class on same subject, that he found brake fluid to work better than any commercial penetrating oils, so I have tried that, and found it works well too. I would love to see comparison tests between ATF and brake fluid. Brake fluid will take auto paint off, if one isn't careful, so use caution in that regard.