2-stroke oil lubricates the crankshaft, rod, cylinder walls and piston pin. Since the fuel passes through the crankcase before it goes into the combustion chamber, there is no oil sump like on a 4-stroke engine. Therefore, the engine has to be lubricated by something. Gasoline is an extremely poor lubricant so oil must be mixed into it or the engine would seize. The oil in the gas coats everything as it passes through the crankcase.