Where's the oil pressure sensor located between the pump output and the filter? I doubt the flow path between the pump output is that long and restrictive. The main galleries are typically pretty big in every engine and that's where the oil pressure sensor is located. I'd think on that Subraus having the capability of putting out 60 L/min of oil flow that they would not have some undersized main oil gallery.
The oiling path is shown in the figure I posted in post #11. The pressure switch is downstream of the oil cooler and oil filter, somewhere between (b) and (c). The oil passages are mostly 11-12 mm, but with some smaller reductions here and there. When pressure at the pump is just above the 102 psi relief pressure, an oil pressure gauge at the pressure switch location will typically read ~75 psi. I think the restrictive oil passages on these engines are a reason why the pump PRV needs to be set so high.
My theory is the main reason for the difference in oil pressure rise is because the oiling system drains down to different levels, and different oil filters may effect that depending on how well their ADBV works. From what you're describing about that engine, it sounds like part of the oiling system can drain down, even if the filter ADBV holds oil in the filter. But if one filter holds more oil in the filter vs the other, then that would impact the time it takes to see full oil pressure on a cold start.
I think that it's a lot more than just the filter that drains. In the first test of the Pentius filter, after it was installed dry, I could tell that both the upstream and downstream galleries were still full. The engine took around 5 revolutions for the light to go out, which was better than any of the tests where the engine was allowed to sit for hours.
With an oil pump output of 12 ml/rev, 5 revolutions is only 60 ml, which should be a lot lower than volume that might be required to fill the dry Pentius filter (which has a volume of around 300 ml, excluding the internals). I'm wondering if that might be explained by the filter maintaining an air pocket for a while.
This video with an oil filter with a clear canister shows an oil filter operating with an air pocket at lower flow rates. The vertical orientation of the filter on a Subaru might allow for a larger air pocket to be maintained since the oil wouldn't need to rise very high up the media to get to the outlet hole. The air pocket does shrink down with pressure after the engine starts, but the pressure in the filter should still be pretty low before the pressure switch sees 2 psi.
Would the taller Pentius filter with its lower restriction media be able to maintain a larger air pocket temporarily, resulting in more oil flow into the main gallery for a given volume pumped by the oil pump?
Maybe the oil light flicker with the Purolator was caused by the air pocket at the top of the filter suddenly shooting through the bypass valve or something.
Those oil passages in the timing cover in Fig. 11 will generally leak, but not everywhere, and probably not at the same place on the same engine. If they leak low down in the engine, I'd expect the oil level in the passages to drain to a lower level eventually. They could leak on the main gallery side, on the oil pump side, or both. I'm not sure the ADBV could do much of anything to prevent this.