From 105
In an earlier section I said that thicker oils are usually needed in racing situations but not necessarily. Remember that a major function of oil is to cool the inside of your engine. In ASTM D 4485 3.1.4: “Terminology: Engine oil- a liquid that reduces friction and wear between moving parts within an engine, and also serves as a coolant.” Since the oil with a viscosity of 10 cS at 212 F thins to a viscosity of 3 cS at 302 F we will get more flow. The pressure will go down some as well. This is OK as long as we have a minimum of pressure to move the oil.
This increased flow will result in increased cooling by the oil. This is a good thing. You would probably want more oil flow in these situations and you get it. The hotter oil thins and this increases flow. The higher flow works harder to separate the engine parts that are under very high stress. It all works out for the better. Higher revving engines need thinner oils. You do not necessarily need to go to a thicker oil while racing. Only experimentation will tell.
In the majority of cases only the manufacturer has time and resources for scientific experimentation, and even then probably only one brand of oil.
As an example
http://www.silkoleneoil.com/techtip6.htm
Am I recommending 0W/20 for high performance engines? Well, perhaps not! The
Pro S 5W/40, which is a 'proper' PAO/ESTER shear-stable synthetic, will look
after a powerful engine better than a heavier viscosity 'cave at the back'
conventional oil, and provide a useful few extra BHP. (On the other hand,
the 0W/20 was very thoroughly developed to give good anti-wear protection.
I think I was on 'Blend 6' before Kawasaki was happy with it!).
I am more confident using say a 5W40 if recommended than experimenting with a thin oil.