what is the pressure in the other cylinders ? I don't think fuel in the cylinder would give a low compression but a higher one as it would fill in any gaps like oil would, there a lot smarter guys here then me, but my first thought would be a blown headgasket, sticky valve, worn rings or broken ring, a warped head, or even a crack between a plug and 1 of the valves all 4 valves are close to the plug. when you pulled the plug what did it look like ? or all other plugs also, John