Pretty much anything will work, if ya let it soak a while. I've used Coca-Cola classic; works great. I've heard of others using straight water. But whatever you use, it's gonna take time to break that chemical bond you gave going on there. Remember, the crank bearings are probably froze up as well, so you may be fighting that resistance. Suspend the engine by the cylinder on some wood blocks, if you can. Then try smacking the piston down & out with a wooden dowel. Use gravity to your advantage.