Typical fuel or condensation stains,more well known as hydrocarbon contamination staining.Usually found in engines that have been run for short amounts of time and not brought to normal operating temp and allowed to set for long periods of time.It is best to bring an engine to temperature,fog the system with a top oil and seal the cylinders for long term storage.It is also advisable to cap the hedders or stuff cloth soaked with WD-40 or similar moisture repellent lube in the collecters of an engine notstored in a controlled enviroment.It is also advisable to remove the rockers to close all the valves to help seal off the cylinders during long storage periods.Even if your car is in a heated garage it is wise to rotate the engine once in a while.