what's the difference between
 

10w-30 and 10w-40?

The viscosity, the 10:40 oil gets to work faster than 1030, that said I ran a CX500 on 10:20 oil for ten years as a despatch rider in London

The viscosity is different. The 10w is the cold viscosity, 10 being thinner than 20. The second number is the hot viscosity, 30 being thinner than 40. So at cold temps these oils will flow the same. At hot temps the 10w-30 will flow easier than the 10w-40.

10-40 is a thicker oil then 10-30.