Running and with the engine/oil warm the running pressure never opens the OPV.
I do not believe that to be true.
If that were true, then adding a shim would have no effect on warm running pressure.
But it does.
I have seen that statement previously, that the OPRV remains closed, but I do not think it is ever "closed" when the engine is running.
The idea that the oil pump is only making 30 lbs pressure, so the valve must be closed, is a completely faulty conclusion.
The spring is understandably easier to compress initially, resulting in pressure losses at low readings.
I have conducted a range of tests involving adding shims to hot engines, etc, and have concluded that the OPRV is always bleeding pressure to some extent, dependant on oil viscosity, temperature, actual pressure reading at a given rpm, etc.
The result was always the same. The pressure reading increased with the addition of a shim, even at idle.
At some point, at idle, adding more preload to the spring will cease to have an effect, as the valve will be held closed throughout, but that will translate through to a higher maximum reading as well, which could be quite high if the OPRV is actually coil bound prior to the relief aperture being revealed.