My Acura 3.2TL-S specified 5W-20 oil. Back when I bought the car new, 5W-20 was still hard to find, so I substituted 5W-30 Mobil 1 synthetic in its i-vTec V-6 engine. Fuel mileage dropped about 1 to 1.5 MPG in commuting service, and the engine felt "rougher" and sounded mechanically louder. The next oil change, I chased down some Royal Purple 5W-20, and the fuel mileage immediately returned to normal, and the engine operation was noticeably smoother and quieter.
I'm old enough to come from the day when we put 20W-50 racing oil in our hot street engines, thinking that it could withstand the stresses better than the then-commonplace 10W-30 oils. As has been mentioned, times have changed, and the manufacturers have set engine clearances and selected oil pump volumes and pressures based on their recommended lube oil viscosities. Change comes hard, but this old codger learned his lesson.
Rusty