Why would one use a motor oil with a higher viscosity than the manual specifies for street driving?
In my opinion, it is detrimental to do so because it takes more power to force a higher viscosity oil through the engine.
Since the oil pump is a positive displacement type, it takes more crankshaft power to run it with a higher viscosity oil.
The volume of oil flow through bearings and the filter decreases with a higher viscosity oil and the stock oil pump.
One could argue that a higher viscosity oil has better resistance to squeeze-out from bearings under high power loading. However, the Coyote Design Team made engine bearing design decisions predicated upon a 5W20 oil viscosity.
A higher viscosity oil is a consideration for an engine with a significant increase in torque and power resulting from supercharging or turbocharging.
Well here are my reasons:
1) I'm supercharged
2) Ford only went with 5W-20 in US/Can due to CAFE (I think it's called CAFE). Since from my research exact same engine in Australia is using 5W-30.
3) Under heat and hard driving conditions after some miles oil usually downgrade a little. So i'f i'm a 30 weight oil at 5K miles it's more like 20 oil.
Plus in my opinion the is not much difference between 20 and 30 oil so the engine won't feel much more stress but it will give just a little bit better protection on a dyno run or under hard driving conditions.