Why does the oil pressure regulator on Mazda engine only have a vacuum tube and no oil return tube?

This vacuum tube is connected to the intake manifold.

Its function is to collect the intake pressure from the intake manifold, and the difference between it and the fuel pressure of the fuel rail is the relative pressure of fuel injection.

The engine needs this relative pressure to be stable in a range, so that the pressure difference between it and the intake manifold is kept at a constant value.

This pressure regulator is generally a diaphragm relief valve. When the fuel pressure of the system rises and the oil pressure at the oil inlet exceeds the combined force of the pre-tightening elasticity of the spring and the air pressure in the spring cavity (from the intake manifold), the diaphragm lifts up, the valve opens, the fuel flows back to the fuel tank through the oil return port in the center of the pressure regulator, and the fuel pressure drops until the valve closes.