Two-stroke oil is a special kind of oil used for crankcase compression two-stroke engines. A four-cycle engine has a closed crankcase, but two-stroke engines in comparison, use the crankcase as part of the induction tract. Because of this, gasoline must be mixed in with the two-stroke oil for lubrication.
When the two-stroke oil – also known as two cycle oil – is burned with the other fuel, there is often blue smoke because the oil is dyed blue so that it can be more easily recognized when mixed in with the gasoline. There is also often a specific odor that is produced by this type of oil.