To follow on from what Stik said, the viscosity/runnyness changes with temperature so W100 will run through a hole faster at high temp than low temp. There is a known change with temperature.
Multigrade oils are designed to be runnier at low temp than they should be and thicker at high temp. Therefore the use of two numbers 10-40 etc. The 10 refers to the viscosity at low temp and the 40 the viscosity at high temp.
The advantage of a multigrade is that it is thinner when cold and therefore circulates quicker. It is thicker at high temp and therefore works better there than a non multigrade.
5-50 multigrade is 'better' in this regard than 10-40.
I'm sure someone more competent will explain all of this using terms like Stokes-Reynolds numbers etc.