"In 5W-30 oil, for example, the two numbers mean it is a "multiviscosity" or "multigrade" oil that is effective over a range of temperatures. The first number, 5, is an index that refers to how the oil flows at low temperatures. The second number, 30, refers to how the oil flows at high temperatures. The W designation means the oil can be used in winter.
A popular belief is that 5W-30 oils, despite their designation, are too thin to protect vital engine parts when they get hot. However, laboratory tests measured the viscosity of oils under high-temperature, high-stress conditions and found essentially no difference between 5W-30 oils and their 10W-30 brand mates. But at low temperatures, the 5W-30 oil flowed more easily."
What this means in practice. is that you choose the lower index number depending on the lowest ambient temperature your car will experience when it has to be started cold.
0W is suitable for cold start temp of below -35 c
5W is suitable for cold start temp of down to -35 c
10W is suitable for cold start temp of down to -30
Nb the above are not exact temperature range but give an idea.(Alfa state that 5W-30 oil for below -20 C and 5W-40 for temps below -15 C)
The max figure in say a 10WQ40 oil (the 40 gives a measure of Viscosity at 100 C.)
A 40 grade oil has a low shear-rate Kinetic viscosity of 12.5 cSt max
A 60 grade oil is 21.5 cSt max
A 10w-60 oil should give better protection over A 10W-40 oil but probably will give slightly less fuel economy as the viscosty is higher.
Both oils in this example would give similar cold starting performance.