With the regular mixed fuel, the oil keeps the piston lubricated inside the cylinder. The piston moves up and down at incredibly high rates of speed. The crankshaft also turns at these same speeds. This high rate of speed causes enormous friction to build and heat up these engine parts. The oil, however, keeps these parts just cool enough to keep running. If no oil is present in the gas, the fuel will still ignite and power the piston and crankcase. However, the fuel will burn at higher temperatures, resulting in piston failure.
http://www.zjhuaruitools.com/gasoline-chain-saw/