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.
Bobby B, a patriotic Long Island Senior Citizen who drives a 2011 5.0 Premium GT Coupe, all black, 6-speed MT82, 3.55:1 differential, Bama 87S Tune, 18" diameter wheels, Pirelli P-Zeros, Comfort & Security Packages, spoiler delete.
God, please bless the USA