BVFD, do the lifters make any sound when the car is at idle? Based on the fact that you don't mentioned it, I will assume that they don't. Odds are, you are looking at a failing oil pressure sending unit. At idle, your oil pressure is going to be pretty low, so, if the unit is loosing its accuracy for example, it can fail in such a way that the gauge will appear to read zero. I would connect an external pressure gauge and see what the system is actually reading. That will tell you if it is a mechanical issue or if it is a gauge issue.

it is also very possible that with the age of the car, things have worn enough that at low oil flows (remember, the oil pump is driven off of the engine and the amount of oil flow is directly related to the speed of the engine), the oil doesn't need a lot of pressure to move through the block. So, you are getting adequate flow, just not a lot of backpressure due to increased clearances.

Also keep in mind that the grade of oil and the type of oil can play into this too. Stepping over to say a 5W-30 oil will cause the system to run at a slightly lower pressure due to the oil being thinner and able to flow through the small cracks better. Also, using say a full synthetic oil can result in lower pressures due to the molecule properties of a full synthetic (synthetics are weighted based on their mechanical lubrication properties, not on their flow properties).

So, connect up a pressure gauge and then rev the idle and see how close the mechanical installed gauge matches to the factory gauge. Then let us know and we can take it from there.

 

less than Thermo's good info. this was an issue when these cars were new too. we put different senders(updated idiot light sender instead of sweeping gsuge sender) in that show mid range on the gauge regardless of pressure unless you have below 10psi. Call Ken at Motorcars