If the engine is gummy, but can be turned by hand, and feels smooth, just sluggish, fuel works well. After all, the oil that's gumming it up came from fuel to begin with. I just did this to an old OS .61FS that has been in storage for over 10yrs. It was gummy, and had no compression, but I could move the crankshaft, and nothing felt "crunchy". After shooting it full of fuel and turning over by hand, it started to loosen up and compression came back. I got it running, and after the first tank of fuel, you'd never know it was in a box for all those years.

If it's totally frozen up, you're going to need heavier duty help. I've used the anti-freeze and crokpot trick, and it works well.

If the engine feels "crunchy", then you likely have rust somewhere, most likely the bearings. Rusted parts have to be replaced, obviously.