What type of measuring device are you using?
Is the gauge electronic or capillary?

There is no reason for the oil pressure to drop once it has climbed through bearing clearance, it might be that electronic (if it is electronic) gauge has a poor sender or a bad connection.
Any oil pressure issues I always remove the sender and fit a known good mechanical oil pressure gauge to confirm.

I am coming to Portugal in two weeks, sorry I won't be travelling with my oil pressure testing kit :0