Originally posted by DennyW
Start your car up. Note pressure. Let it run until warm. Note your pressure. Now, sitting still, raise the idle up to a fast idle, while watching pressure, and noteing any sounds you hear. If you hear a knocking sound then, that would be a rod knock, most likely. Hows the oil pressure? Then, let it idle, put it in gear. Note oil pressure. Now, while holding brake, put a load on the motor, like torqueing it up (power braking), and note the oil pressure. If you hear a thumpping sound, and pressure drops, it's the mains. Last, it you cruise on the highway, while watching oil pressure. If it starts to drop as you gain speed, you most likely have a pickup tube that is clogged. If it comes, and goes on the oil pressure, it's probably sucking air.
this is sound advice