Low oil pressure

When driving at any speed after the engine has come up to normal temps, I've noticed my oil pressure is about 6psi. Yes 6psi, not 6 bar. Its pretty low. In city, at idle, on the freeway, doesn't matter. It sticks at 5-6psi no matter what the revs. However, when the engine is cold, it will be at max pressure. While heating up and driving, I've noticed it runs as normal, but slowly drops in pressure and fails to increase pressure as its normal constant pressure is low. In other words, if its at 1.5 bar at idle, it will go above 2 bar when I accelerate, but when it drops to 6psi at idle, it won't increase at all. My oil was at least 5 quarts low (just bought this car and its a 1990 3.6), but filling it up had no effect on the pressure. Is this just a sign of a failing oil pump? Or is it possibly a failing pressure gauge that stops working right as it heats up?