My understanding is the 0w-20 oil gets up into the engine parts and valvetrain quicker. This is important with the many engine starts and stops in a hybrid.

I found some information that says the 0w-20 synthetic oil will protect the engine from -53°F to 400°F

I'm not sure why, but somehow the thinner oils slightly increases the gas mileage.

Evidently the 5w-20 dino oil works fine or toyota would not put it in their engines.