I use Kroil for cleaning firearms, it removes copper and powder fouling from the bore quite well.
It's too expensive to use in quantity on old cars or boilers.
In the old car restoration business several different studies have been done and the results are that whie all the commercial penetrants do work, the best is the ATF/solvent mix, and it's better by a wide margin. The studies use rusted fasteners in a controlled study, and use the 'break-awy torque needed after an application of different penetrants after various times of soaking.. Neat studies, and the stuff does work..
All the commercial penetrants are kerosene with a perfume added. Kerosene is close to Diesel and Jet A,, and ATF is just a very thin hydralic oil, around 0wt .
The solvent tends to do the creeping into the tight, rusted crevases, but it brings along the disolved ATF, then the solvent evaporates, leaving the ATF as a lubricant.
ALL commercial penetrants are flamable, and caution should be used. Kerosene burns very well, regardless of the can it comes from.. I used to use WD-40 as a starting fluid in my chainsaw.. works great.. IE: very flamable.
Like Berlin, I've found best results from using heat, a torch of some kind, then cool the rusted joint with ATF or even regular motor oil, it fumes and will ignite if there is open flame.. and it stinks.. but the oil is drawn in by the cooling of the rusted joint, and eventually the parts break free.
I mix the stuff in a 1-gallon solvent can.. I had a half gallon of acetone remaining in a 1 gallon can, i added 2 quarts of ATF, and it's lasted a long time.. I have used the old style 'finger pump' oil can to dispense it, but you can use a plastic pump bottle like from '409' or the many cleaning products.. the spray mechanism eventually fails. but you were going to thorw the spray bottle away anyway? right?
Anybody workng unde a car with lots of penetrant gets a snoot-full of fragrance and atomized solvents.. well I get a headache, not a pleasant 'high'
Good luck with your rebuild project,, looks like a good find, and it'll keep you home at night for a while.