Make that MARVEL MYSTERY OIL, squirt it in, turn the engine by hand, let it sit, repeat every few days for two weeks more or less. This is an old family reciepe that has worked out well. It will help free a stuck ring if there is one. Expect some white smoke when the engine first starts.

My father had me "rinse" out an old engine with straight diesel fuel. Actually ran it briefly to get the diesel fuel running thru the lines, then let it sit in the oil pan for a few days before draining it out. Probably didn't hurt anything but there might be a better way to clean out the crud now. I'd probably add some engine oil to the diesel if I were going to try that now.