Ok. Lets clear things up here. Too many codes and number floating around.
First, an explanation of terms:
Viscosity Index (VI)
The rate of change of kinematic viscosity with temperature is the viscosity index. A high Viscosity Index indicates a smaller decrease in kinematic viscosity with increasing temperature.
The SAE had originally drawn up a scale to measure VI of oil, with 0 being the worst and 100 the best. However, with the improvement in technology, oils have gone over the 100 mark, hence the basic rule is that the higher the VI, the more stable the oil is over a wider range of temperatures. That is a good thing.
kinematic viscosity
Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity.
The higher the kinematic viscosity of an oil, the thicker the oil film that will ding to a metal surface. The higher the boiling point of crude oil, the greater its kinematic viscosity.
So what this means is that at a lower temperature you need that number to be lower, as that means the oil can flow to the area's required much sooner.
Now lets take 2 examples of 100% synthetic oils and compare their ratings as provided by the manufacturers themselves:
1.
Motul 8100 X-cess 5W-40: For petrol & diesel engines, meeting API/CF standards
2.
Mobil Delvac 1 5W-40: for Heavy duty diesel engines, meeting API/CI standards
Lets compare the specs (Motul / Mobil):
SAE Grade: 5W40 / 5W40
kinematic Viscosity @ 40șC: 81.1 / 102
kinematic Viscosity @ 100șC: 14 / 14.8
Pour point: - 39șC / -45șC
Flash point: 228șC / 226 șC
Viscosity Index: 173 / 151
As can be seen from the above example, Delvac 1 is a thicker oil which does not flow as well on starting up. Once upto temperature, they seem almost equal.
However, what is important is that the VI for Motul is far higher than that of Delvac, which means that Motul is a far more stable oil which will give you more predictable performance over a wider range of temperatures. Motul also has a marginally higher flash point.
None of these oils are bad, but for passenger car use, I would much prefer the Motul, as can be seen from the numbers.
Moral of the story: Don't do blindly by the ratings on the can.