Their molecular distribution is more streamline and small but you are completely wrong about the quoted comment..10w30 or 5w30 oils were formulated due to extreme emission control regulations in the EU and states which resulted in their origin..They increase fuel economy marginally but are not the best at engine protection..
High mileage oils are available in 10w60 viscosities the are the most expensive synthetic lineup with a drain interval of 50K +..Does that mean they are not efficient?WHy do they need to formulate oils in this range??
A thin oil flows fast at the expense of less viscosity..Thus reaching engine parts in time but with reduced friction control..
Thicker oil takes a little more time to reach different parts but it has high friction control(It is thick and has high shear control) and protects the engine better than a thin oil..
Any ways you wont find a marginal difference in 5w30 or w40 but i would certainly prefer the latter..
an engine flush performed using a mineral oil every 3rd oil change would keep the engine in nearly same protection as a synthetic..The only difference is in the drain intervals..