Better yet, put a capillary oil pressure gauge into the drilling. I was running around in my last car with near-zero oil pressure because I did a test with the capillary gauge and diagnosed a problem with the instrument voltage regulator. This should give you a good idea of the pressure in the main oil gallery, which is what feeds the crankshaft bearings. Slow pressure build-up here suggests the problem is up-stream, in the oil pump or filter.
Oil pumps have been known to come loose, allowing oil pressure to bleed away. Only two bolts hold it in place. If you are going to drop the sump to check the oil pump pick-up for clogging that would be another thing to check. While the sump is off you can pop off the main and big end bearing caps and check the condition of the bearings.
Hydraulic tappets can "bleed down" if they are worn and then take time to "pump up" again, but as ZF says, this would normally be associated with some rattling noises at initial start-up. I am not sure whether 30,000 km constitutes a high mileage for a 16V engine, but the volume of oil required to fill the tappets is quite small, so I would not have thought that this would affect the residual pressure in the system.
Oil fiters are cheap to manufacture; it is always possible that you have fitted a replacement that also has a poor non-return valve. It can happen
At the expense of changing the oil again, it might be worth a new (rather than a factory spare, maybe from a faulty batch) replacement.
Good luck with your investigations!