Before understanding when it is appropriate to use two stroke oil, you will have to know what the oil is made of. The oil-base stock is typically petroleum, vegetable, semi-synthetic or synthetic oil. This is added to petrol or gasoline at a fuel to oil ratio from 16:1 to as low as 100:1. For lower emissions, the new two strokes use synthetic oil.
The most common time to use 2 stroke oil is in a crankcase compression two-stroke engine. These engines are different than their four-cycle counterparts and need different oil as a result. A two stroke engine is lighter weight and does not have a closed crankcase. Instead, the crankcase is used as part of the induction tract, meaning gasoline must be mixed in with the oil at some point during the process.