The first number indicates the viscosity of the oil at a cold temperature, while the second number indicates the viscosity at operating temperature.
They are all a 30 weight oil at operating temperature. They only differ when it's cold outside.
Just becuase it's a thicker oil, doesn't mean it will last longer. If you are running mobil 1 5w30, you are good for 5-8k miles. If it's their new "extended performance" mobil 1, then 15k miles. If you want to get a good synthetic oil that will last longer then that, check out amsoil 5w30. Good for 1 year or 25,000 miles. Their 0w30 is good for 1 year or 35,000 miles. Use a high quality filter if you want your present oil to last longer...stay away from fram lol. Use a mobil 1 filter with your mobil 1 synthetic oil.