I'm going to go out on a limb and assume this is a cylinder deactivation (AFM) engine. The engine oiling system on these is divided and regulated in two parts; the main oiling system, including crank mains, rods, and cam bearings are regulated at whatever the oil pump mechanical bypass is calibrated to. The AFM assembly under the intake, including the sensor, is regulated to 55-75 PSI by a separate mechanical regulator that screws into the oil pan. Symptoms when this second regulator goes bad usually are that oil pressure is fine cold, but as the engine warms up and the oil temp increases pressure continues to drop, eventually reaching the threshold at which the ECM sets the P0521 code. (It's around 15 PSI, can't remember for sure.) GM's test procedure to verify oil pressure actually includes 2 mechanical gauges; one in place of the oil filter and one in place of the sensor. Long story short, if you're checking pressure at the sensor with a mechanical gauge, you're not reading base engine oil pressure. Also, the different filter may contribute to differences in oil pressure readings, but in my experience likely isn't the root cause of your problem.