Originally Posted by
aaronng
If the viscosity rating is the same, then both oils will have the same thickness. If you have a group 4 that is thinner, then the viscosity rating would be lower.
The difference between a 5w-30 and 0w-40 is small enough that you wouldn't be able to measure the power loss on a dyno as the dyno is not sensitive and accurate enough.
Viscosity is only a measurement of the resistance to flow. It does not measure how slippery the oil is. For example, water has a very very low viscosity compared to oil, but is not really "slippery". Oil on the other hand is much higher viscosity and you can see with your own eyes that it is thicker than water. But when put to use as a lubricant, it is more slippery than water, hence giving it better protection properties.
One unknown difference between a Group III and Group IV is that a Group III will tend to cling to metal surfaces for longer, whereas Group IV tends to be less attracted to metal. Not an issue if you drive daily though, only worth taking it into account if you only drive once a week.