Use the correct "tool" (chemical) for the job. That is this stuff, which is not interchangeable with any other penetrating oil or fluid—you'll know it as soon as you smell it.
Also realise that you may or may not be able to get this valve to unstick and stay unstuck. After many decades and thermal cycles, sometimes they're just done, especially this early (pre-'71) type.
A working heat control valve is a very good thing to have, and not just if you live someplace cold. Many people don't clearly understand why the valve's there. "Rip it out/block it off/weld it shut" etc is advice worth smiling, nodding, and ignoring.
There used to be rebuild kits (shaft, plate, counterweight, counterweight key, thermal spring, antirattle springs, stop dingus) but they've been unavailable for many years. At this point a manifold that's used enough to have a perma-stuck valve is also likely to be warped and/or cracked; the fix is a new manifold—see here for particulars.
And since you've got the manifold off, see here for hardware info.
