BG In-force is the oil I would use. We removed multiple bolts from a tractor manifold by hand with it lol.

 

Keep in mind that if you have a firmly stuck header bolt broke off flush in the head, it most likely cannot be removed.

Instead the easiest way is to drill out the center of the broken bolt and re-tap it to a smaller size with a bottoming tap. I had to do this during my header install, and ended up making it 1/4" instead. You are doing the opposite of a heli-coil basically.

 

If its broke off flush then center a nut over it and weld the bolt stud to the nut, use a nut with a smaller inner diameter then the outer diameter of the stud so you don't damage the head.