Though necessary for your vehicle, engine fluids can become dangerous, especially when they get into and contaminate another fluid. When one kind of fluid mixes with another, it reduces the effectiveness of one or both of them, depending on how the contamination happened, and can cause all kinds of havoc in an engine. Oil in your coolant (or vice versa) is probably the most common and well-known kind of bad news fluid mixing, but what about others? What about, say, gas in your oil?