I put these together for a member asking if they should change viscosity grade, hopefully they'll be helpful here too;
There are two ways to control an oil viscosity (thickness);
Use longer molecule base oils. There are weak attractive forces between molecules, as the molecule gets longer, there are more forces which makes it harder for one molecule to flow against the next, increasing viscosity.
Use a viscosity modifier. These are additives which close up at low temperature and open at high temperature, when they are open they have strong attractive forces with the base oils, but when they are closed they don't. These allow oils to stay low viscosity at high temperature (good for cold starts) and not lose as much viscosity at high temperature (good for wear).
When oil viscosity is described, most people use kinematic viscosity (usually expressed as KV100 - kinematic viscosity at 100 deg. c). This is a measure of oil flow and is comparable to pouring the oil by hand. The viscosity modifier has a big influence on viscosity here, so there are massive differences in temperature response and viscosity grade.
This is where the suggestion of increasing viscosity grade for hot engines come from. If you were using a 5W-30 when the engine runs at 100 degrees, and increase to 125 degrees would mean using a 10W-50 to maintain the same viscosity.
However, the kinematic viscosity (gravametric flow) doesn't matter in an engine, because in an engine the oil pressure and contact speed of the surfaces puts much more energy into that oil than simply monitoring how fast it flows with gravity. These forces cause temporary shear of the viscosity modifier which means the influence of the viscosity modifier decreases. This viscosity measurement starts to look like just the base stock viscosity. This is measured using a High Temperature High Shear measurement (HTHS).
You can see with this relationship there is much less effect of temperature on viscosity.
Furthermore, I have deliberately picked two oils with very similar HTHS response but different viscosity grades. This is because an oil or additive company can elect to meet a thicker viscosity grade one of two ways:
1. put longer molecule base stock in
2. put more viscosity modifier in
There is a general trend towards using the same type of base stock across different viscosity grades. This is to avoid logistical complexity of the oil blender needing to carry many different tanks of base oils. So this means that sheared viscosity is often quite similar between different viscosity grades, and leads to there being very little reason to change viscosity grades for hotter engines.
Thanks for that 
