This is one thing I don't understand. When the engine is running, the oil is pumped by the oil pump towards the upper part of the engine (cylinder head assembly). I know there are some galleries inside the head that helps oil to fall back into the oil pan but suppose an engine is turning at 3000 - 4000 RPM. At such engine speeds, the oil pump is spinning so fast that sucking the oil upwards will happen faster than the oil going downwards to return to the oil pan. When this happens, the level of oil may go down and the oil pickup may suck air instead of oil. How has this issue been addressed by car designers?
The galleries only hold a small amount of oil, between 0.5 and 1 litre roughly.
As the pump forces oil into the galleries, that same amount comes out and returns to the sump.
The sump holds 4 or more litres so there is always enough for the pump to draw on and also enough when the level changes due to hills or braking etc.
The other thing the oil does is to carry heat away from the bearings etc and move it to the sump where it cools - in some engines this sump cooling means fins on the outside of the sump or even an engine oil cooler can be fitted.