Ok.. Studebaker my buddy was helping get going. flathead from the 50's... froze up.. we mixed the ATF acetone mix penetrating oil and filled the cylinders from the spark plug holes. The beauty of this is that it is a greats penetrating oil but... it is also cheap. you got nothing to lose. Anyway it worked.. to be safe once it broke loose he just turned the crank 90 degrees and filled the holes again.
Fired right up a few days later and he ran it for a minute then drained and changed the oil which was awful the thing smoked a bit for a while after that but really not bad at all now.
i've used that mix many times to get things to loosen up 'round the shop.