Understanding Oils

[Crazy Dazz]

Ok, so my basic understanding of engine oils is this:

SAE J300 defines the grades and viscosities associated with them.
With a multi-grade oil, eg "20W-40" the oil is capable of behaving like the winter weight SAE20 in cold weather, and like SAE40 at operating temperatures.
Generally the higher the number, the greater the viscosity.
So, in theory at cold start you want lower viscosity so that the oil flows quickly??
And at operating temperature you want higher(?) viscosity to provide better protection?

I think I have the above correct, but I'm still puzzled.

Am I correct in thinking that a bigger range is better? eg is 10W-40 better than 10W-30?
OR, if I put in 10W-40, will that be TOO viscous at operating temperature?

And what of the Winter Weights? For example I have an engine that takes 0W-30
"0W" defines Cranking & Pumping Viscosities at -35 & -40 deg C! WTF do I care, since I would be dead?
I assume that number is specified to give certain characteristics at more realistic temperatures, but don't know what they would be.