If you're going to be doing this for a period of months, I would guess most any solvent type liquid or penetrating oil would do the job. The key to most all of these "soak treatments" is time. You're giving it plenty of that.
Regardless if it's threads on a frozen bolt, or gunked and carboned up piston rings. Just be sure to keep reapplying it. The wetter the affected area stays, the better. And the longer it stays wet the better.