Engines require oil FLOW while oil PRESSURE is related to flow and viscosity. See YouTube: The Difference Between Pressure and Flow

The pressure measured by an oil pressure gauge is typically at the oil pump or in the main oil gallery.  Once the oil reaches its destination (tappets, bearings, etc), its pressure is essentially atmospheric (ie, 0 psig) or close to it so the measured oil pressure is what the oil pump develops in response to downstream flow restrictions.  The difference in pressure between the pump and where the oil leaks out of the bearings is the pressure loss (ΔP - Delta P) and ΔP varies with the square of flow.  That is, if flow doubles, the developed pressure quadruples.  See Darcy–Weisbach Equation.

Oil pumps are positive displacement, which means their flow is directly proportional to speed.

External Gear Pump

External Gear
Positive Displacement Pump
Internal Gear Pump
Internal Gear (Gerotor) Gear
Positive Displacement Pump
Wikipedia Gear Pump Animations


Since oil is incompressible, a positive displacement pump can develop extremely high pressures. Excessive pressure can cause oil filters to rupture or oil pump gears to shear so hydraulic pumping systems always have a pressure relief valve (pressure regulator) to limit the pressure developed at the pump.  Excess pressure is bled off by the relief valve by diverting some oil flow back to the sump so you're better off keeping the oil's viscosity to the lowest that protects the bearings.  Excessive viscosity wastes engine power from frictional losses, which is another good reason to keep oil viscosity as thin as possible and as thick as necessary.


Dual Filter Engine Oil System


As you can see by the following photo, engine oils become more viscous (thicker) as they get colder but the 0W-30 oils flow far better than 15W-40.


Engine Oil Viscosity Comparison