If you are getting dark exhaust, look for a leak from the exhaust port to the muffler. That can turn your exhaust dark. The fuel I am using is dark green, so the exhaust oil is dark anyway. I have always emptied the tank into the fuel can. Another thing that can cause dark exhaust is the gasket or shim at the back of the crankcase missing. If you use an electric starter, pushing on the prop causes the back of the crankshaft / bottom rear of the connecting rod to scrub against the back of the crankcase. Get a fresh drop of the exhaust oil on your finger, rub it gently, and look carefully. (If you had my vision, doubly carefully). If you see very tiny particles of metal, that should be your problem. Or, it could be bearing starting to go bad. Try the easy stuff first, then go deeper if you have to. Check the engine forums, there are a bunch of very experienced people in there, and some of the major manufacturers reps. But just reclaiming the fuel is not a problem.