I am a bit unclear as to the problem you are asking about. Is it that you have noticed that the oil pressure in flight, after the oil has thoroughly warmed, is now lower than previously experienced? If that is what you are seeing the problem may also be trash under the oil pressure relief valve seat preventing it from fully closing to maintain pressure.
Now, that trash could be indicating something serious, such as cam spalling or some other part throwing off metal, so the end result, pulling the engine, might be the same. Never-the-less you might want to pull the valve, clean and inspect the seat, and also closely examine the filer (if you have one) for metal particles. If you only have a screen pull it also because, while not as good as a filter, it will also occasionally stop bigger pieces of metal. If you don't find anything under the seat, don't be surprised if the pressure is restored on the next flight. Sometimes those little pieces causing the mischief drop off or out of the way when the relief is pulled and you never notice them.