My son's son to be father in law gave me a Cub Cadet to mess with. The engine, a 19 hp Kohler Courage, had locked up with a little under 200 hours on the engine. I disassembled the engine. The piston was stuck to the bore. There was a dark brown goo more akin to roofing cement in the bore and all over the piston. There is also nasty brown sludge in the bottom of the crankcase. Here are a couple of pictures.
Bottom of crankcase
In the process of trying to clean the crud off the piston, I tried a variety of solvents. I tried gasoline, acetone, toluene, xylene, lacquer thinner, and naptha. Nothing dissolved it. After giving it a little thought I tried water. It dissolved instantly!
I daresay that someone put sugar in the tank. I know some will say that sugar does not dissolve in gasoline but sugar will certainly dissolve in ethanol and since much of the gasoline is 10% ethanol, my conclusion is sugar in the tank.
To be honest I think someone dumped a Mountain Dew or green Gatoraide into the tank. The green coloration on the piston and light green deposits on the head suggest that is a possibility.