Few graph here simply give you an image that how common viscosity range act in the engine.
2.8% increase in power output just for changing 0W40 to 5W20. It is fantastic.
Suprise! Despite 5W20 oil consider as a thin oil but in fact, in real world, the film thickness is much stronger than 15W40 or 5W40 oils.
And great fuel economy gain by thinner oil like 5W20