flying with this a/c since then I see that the oil pressure is closely linked to engine revs, always within the green, but only just at the low end.
The engine oil pump, is, of course, a hydraulic pump, whose output (flow and pressure) is in direct proportion to engine speed. The oil pressure relief valve is designed to assure the minimum required pressure is maintained, while excess oil pressure is dumped harmlessly back into the system at low pressure. Other factors affect the oil pressure, in particular oil viscosity vs temperature. It's hard to maintain high pressure with a very low viscosity oil in a [somewhat] leaky system.
So if the oil pressure indication is obviously changing in proportion to engine speed, though remaining in the green the whole time, you should understand that things are okay, though not ideal. For ideal, the oil pressure indication will remain constant, and ideally in the higher portion of the green at all engine speeds. An engine whose oil pressure varies with engine speed within the green is going to get you home, but I would not buy it! The one proviso to that is viscosity breakdown in the oil, which I described earlier. The engine was perfectly fine, the oil was no longer functioning.
FWIW, a retired Continental Engineer told me, in the context of my O-200, that as long as the full power oil pressure was at least 30 PSI, and the idling oil pressure was at least 10 PSI, the engine would run safely. This corresponds to what the type certificate for the engine, and the C 150 flight manual state. Obviously they allow for pressure variations with engine speed.
The engine relies partly on oil pressure, but more on oil flow. The oil flow provides a film of lubrication, and carries away excess heat, and the pressure provides some resistance to metal to metal contact against engine internal mechanical forces, and provides the required pressure for the hydraulic valve lifters. If there is a concern about low oil pressure (as opposed to no oil pressure), and the quantity is sufficient, the first, and lowest cost check is to change the oil, and run the engine again to see if the problem is improved.