When there are two numbers it is multi viscosity oil. It has an additive so that when it heats up it acts like a higher viscosity oil. Something about cross linking of the polymer chains that increases the viscosity when it heats up. So 10W-30 acts like a 10 weight oil when cold but a 30 weight oil when hot. 30 weight oil is always 30 weight. So when it is cold it is too viscous which is not good for the engine. Multi weight oils are a great invention and should always be used in my opinion. The only place that straight 30 weight oils should be used are in small engines which run really hot and can break down multi weight oil and you don't run them in the winter anyway.