I use Marvel Mystery oil in cases where an engine sat a long time and things are stuck due to old oil getting thick, esp stuck rings. I wouldn't fill anything more than just the cylinders.

MM Oil is slightly acidic in nature and that explains how it dissolves gum too. But Blaster probably works better but I've not used it b4 (because I didn't have any).
But if you have really rusty cyl walls that is entirely a different matter as it builds a bond onto adjacent metal. Good luck and let us know after the head is off what it looks like.

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