Another item which might affect your diagnosis is that the oil pressure instrument on the dash may not actually be a gauge, but an indicator. Some models, like B-body cars and K-body MPVs use an oil pressure "sender" which is actually just a switch. The I/P uses a resistor to position the oil pressure "gauge" at a predetermined point of sweep just to indicate that sufficient oil pressure is present, and doesn't actually vary to indicate relative oil pressure.
If your oil pressure instrument doesn't vary from dead cold startup to full-hot idle pressure, it may be one of this type. This is why a mechanical gauge tapped into the system is important for diagnosis.
Also, read your owners manual carefully. There is usually a sliding scale for determining the appropriate oil viscosity under various temperature ranges. Despite the oil filler cap displaying the "5W-30" script it is often appropriate to use 10W30 in temperatures above 25°F - Right in the owners manual.