For extended run times with few cold starts, and the lower rpms that most vehicles run at, oil viscosity isn't as important for mileage.
If you are cold starting often with a 1.6l 4 banger and then driving at 3500rpm at 65mph for 20 minutes twice a day, then a jump from 0W20 to 15W40 would be pretty noticeable for mileage.
I think the extra drag of going from 0W16 to 0W20, in your usage will be undetectable. For Toyota, a 0.5% difference in lab testing is significant, but I think the test cycle they have to use isn't representative of your usage pattern.