2003 forrester 2.5l non turbo. has low oil pressure light on after engine warms up at low idle (around 800rpm). light goes out when revs are above1500. drive for 10 minutes and light comes on when revs are higher but only when you let off the accelerator (engine under load at 2000 rpm light is off, foot off gas pedal revs drop to 1500 and oil light comes on. oil is clean and filter is good. no visible oil leak , no oil on driveway and no oil around oil pressure sensor or anywhere up top. any ideas would be appreciated.

that's a real issue. that light comes on at 2 or 3 psi ! is the oil pan dented?

 

how many miles on the car? any engine work done? there's an o-ring in the pan that I think can cause problems with pressure. Also, it's possible, though rare, the pick-up tube is cracked. maybe the pump has loose backplate screws?

 

there has been no recent engine work, the engine has piston slap but i was told it is no big deal. 190000 kilometers (120000 miles). no dent in oil pan.

That light comes on when it's basically too late.

 

You can put a pressure guage on it and see what the pressure actually is. It may be a faulty pressure switch.

If it is low pressure, check the pump and the pickup tube first. Beyond that you're looking at bad main and/or rod bearings. Those will also be accompanied by silver swirls in the oil.