One reason why you want to see some pressure on the gauge: The oil pressure gauge, or warning lamp, gives only the pressure at the point where its sender enters that part of the pressurized system — not everywhere, not an average, nor a generalized picture of the systemic pressure. Don’t trust it to guarantee bearing condition. Despite the frequent comparison to hydraulic engineering theory, this is not a “closed system” in which oil pressure is balanced and identical throughout the engine.
All engines are open-ended hydraulic systems, because the oil returns to the pan by a series of controlled leaks. The bearings farthest from the pump always have the lowest pressure because of the number of leaks between the pump and that bearing. Excess bearing clearance increases the pressure lost between the first and last bearing in a series. Depending on its condition, an engine may have acceptable gauge pressure, and still only have 5 psi pressure at one connecting rod
