let me explain further.

if at 3000 rpm, the pressure is 80 psi with a 50
and after filling with a 40 wt, the pressure is still 80 psi
then the same or more oil is flowing through the engine.

Why more?
Imagine trying to push molasses through a system. The system could be bottle filling system. Whatever you want to imagine.
Then picture using the same effort (pressure) to push honey or maple syrup through the system.
That same pressure is going to flow more maple syrup and flow it further.

Keeping sufficient oil to float the bearings etc requires both pressure and flow (psi and gal/min)
The faster the engine spins the more volume per time is needed at the bearings.
So the goal is to have the rising pressure increase the flow as far as possible up the rpms.