[Steve Tinker]

In addition to what specific readings the CEL is going to tell you, low oil pressure warning light illumination is usually caused by:
Oil grade too thin.
Worn engine bearings.
Worn oil pump.
Faulty oil sender unit.
Idle revs too low (should be approx 800rpm)
If you have low oil pressure @ normal 800rpm, there is a good chance that you have low oil pressure at all revs.
The only real way to check is to hook up a mechanical oil pressure gauge to the engine to verify the correct pressures at running temps and revs. I wouldn't dismiss this potential problem out of hand, although an oil change with the correct viscosity oil is a good starting point. And check the oil filter internals for debris.....