1996 Jeep Cherokee Black fluid in coolant reservoir, low cy

Recent purchase, drove 150 miles toward home with CEL on, no other warning light ever came on. Engine ran alittle rough, not bad. Fluids were full before trip (they overfilled oil). Stopped for a break, there was a thin black fluid pouring out of the radiator/coolant reservior (there's a rubber open spout), was bubbling and fuller inside, now its empty, main radiator cap has ink-like black under it. Did not overheat, no exhaust smoke, no coolant in the engine, engine oil has a very slight gas-like odor (injector cleaner we put in?), Lost at least a quart of oil, no or little lost transmission fluid (blackish color). Incomplete info given by the shop: CEL codes - "downstream 0202" (injector circuit#2?), Random misfires, Misfire on Cylinder #3. Cylinder compression numbers: 60, 50, 40, 90, 80, 0 (0 changed to 60 by different shop clerk). Said radiator is good but cooling system caused the problem, overheated before purchase. "Its a blown head gasket or bad valves." Are these codes causing or result of the oily black fluid? Car still runs/sounds the same. Could they be assuming it's a blown head gasket when its an engine oil cooler/radiator leak? Any other causes of black fluid in radiator? How can it be fixed?