Nissan Repair: low oil pressure, wild goose chase, nissan 300zx

Question
I have a 1986 Nissan 300ZX normally aspirated with 144k miles on the odometer,..With the engine cold and before reaching normal operating temperatures the oil pressure will go to 45lbs..at normal driving speeds,..(around 2000rpms),.once normal operating temperature is reached the oil pressure will drop to somewhere in that 15-20lb range at operating speeds(2000rpms),..at idle once normal operating temperature is reached the gauge will lay on zero and occaissionally the red warning light will flicker,..some suggest this is the check valve in the oil filter mounting base,.while others will say you need to replace your rod and main bearings,.and still others say your oil pump needs replacing,...i'm prepared to do all of these to correct the problem, but would rather not be on a wild goose chase,.I also fear that the previous owner frequently let the car run low on oil due to leaks,..the car has no visible signs of burning oil and has average compression on all six cylinders,..what do you feel would be the correct diagnosis?

Answer
Lee,

I would replace the oil sending unit first to make sure it is not just the sensor.  Ten pounds of oil pressure for every thousand RPMs is normal.  But, the light coming on at idle is a bad sign.  So, change the pressure sensor and if the light is still coming on you need to probably change the bearings or the oil pump.  No way to know which until you remove the pan to get to the bearings.