Any oil is better than no oil.
Don't let the low winter (or more correctly, 'cold') number fool you. Oil lubricates just as well at 10 weight as it does at 40 weight.
The big thing is at start up. You WANT a low viscosity oil to establish a boundary layer as quickly as possible. That's done a lot faster with a lower viscosity oil.
There are some very good articles out there that explain multiweight oils and why a multiweight is better (and a synthetic is best) mainly due to start up and (while not technically correct) 'dry' bearings (as in not enough oil to create a sufficient oil film).
It really does get technical quickly, and its not as simple as the t.v. commercials make it out to be. But it is very interesting to read.
I completely agree that 30 weight is just as good an answer as 10W40.
I was surprised to learn that oil never 'wears out' (aka loses its lubricating ability) but its the ADDITIVES that get 'used up' and require the oil to be changed. You can filter the oil clean, but without the detergents/friction modifiers, anti-oxidizers/anti-foaming agents base oil won't last long. Its really amazing what they can get modern oil to do compared to when they ran straight mineral oil.
