To thin oil weight, If it calls for 10W-30 and someone put in 5w-50 in older engines after it warms up the oil pressure will be low.
Crank bearings are worn, when the oil gets warm it will flow around the bearing leaking back into the oil pan not getting pushed up to the top of the engine and give low pressure.
warn oil pump, low oil level,blocked oil passage

Those are the most common problems other then a bad pressure sensor.