On the face of it, automotive oil (used in regular passenger cars) and dirtbike oils are very similar. However, before you rush in and use automotive oil in your dirtbike, you should know a couple of facts.  Firstly, many (but not all) dirtbikes have common sumps so the motor oil is used to lubricate and to cool the transmission. In regular cars, the transmission is lubricated by ATF fluid which has frictional properties that are needed for the way it works in the transmission. Regular engine oil cannot do this. Therefore, the engine oil that you use in a motorcycle, has to be able to lubricate the transmission and must have the same level of friction modification that a typical car oil does. It must also be able to prevent lock-up and slippage. For this reason, it will not contain the friction modifiers that automotive oil would have.
Secondly, motorcycle engine oils are designed to break down viscosity quicker than automotive oils. They, therefore, give very good protection against viscosity loss.