2-stroke oil uses a different additive package that is designed to not leave excessive residue behind when it burns. This is because 2-stroke engines burn oil by design, whereas automobiles keep the oil more or less separate from the combustion. 2-stroke oil is either "ashless" or "low ash" which means the detergents and anti-wear compounds will burn off either completely or almost completely. Automotive oil may use additives that do not burn such as metallic compounds.
If you run regular automotive oil in your 2-stroke, it will probably lead to an accumulation of deposits in your engine. This is bad for performance and could eventually cause the engine to seize.