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

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


Positive Displacement Pumps


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 psi) or close to it so the measured oil pressure is what the oil pump develops in response to downstream flow restrictions.  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.


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