The viscosity at a 40c should low to give start up protection and the viscosity at 100c should be high for good protection when hot.
The VI is the viscosity index it is a measure of the range of viscosities the oil spans i.e. a high VI number is good. Synthetic oils have higher VIs than mineral oils.
The VI of mineral oils depends on the quality of the oil base stock and the additives oil, the oil additives break down the the VI of mineral oils is reduced with use.
Even the cheapest supermarket Synthetic oils have superior base stock to mineral oils and have very low quantities of additives added.
The Pour Point is effectively the low temperature at which the oil becomes jelly and will not pump through the engine. A low Pour Point is essential is severe winters.
Synthetic oils tend to have much lower point temperatures than mineral oils.
Below is a good link for oil info
http://www.kewengineering.co.uk/Auto_oils/mineral_or_synthetic_oil.htm#_Toc240461876
[Edited on 15/9/13 by britishtrident]
[Edited on 17/9/13 by britishtrident]
[Edited on 17/9/13 by britishtrident]