You really need to put a gauge on it to see what the actual oil pressure is before firing the parts cannon.
I agree. Low oil pressure is not something any of us ought to be ignoring, or over-reacting to. I picked up a set of plenty functional oil pressure gauges from Freight Harbor. One kit has one gauge, and a gauge pressure relief button you need to press to relieve the highest oil pressure level attained prior to pressing the button. There are uses for that feature, but I feel that poses the disopportunity to overlook a low pressure condition if the pressure relief button is not pressed n order to get an accurate reading under specific conditions. So, I then pickup up a kit with two gauges, one for very high pressures, the other for lower pressure levels. Neither has the pressure release button, you get a direct pressure reading for every moment the gauge is connected.
Both kits include several different brass and steel fitting adapters that should allow you to connect to pretty much any oil pressure test port you come across in an automobile. I have used my gauges to verify the oil pressure gauges on our different pony cars were displaying accurately, and to adjust the Throttle Valve Pressure for the AOD transmission I swapped into our 1973 Mach 1.
As for the Oil Pressure situation you are facing, the general rule of thumb I follow is to look for 10 PSI per 1,000 RPM the engine is running at. If I happen to have 30 PSI at idle, as opposed to 10 PSI or so, I do not fret. I just increase the RPM to see if the PSI reading stays below 60 or 70 PSI. At that my real concern is to make sure I do not hit 100 PSI at any RPM level, as that indicates a stuck oil pressure bypass valve, which could damage seals or make the oil filter bulge out (at best). If you are getting 10-20 PSI at idle it is sufficient for engine protection. But, the PSI should be going up as you increase the engine RPM, pretty much following the 10 PSI per 1,000 RPM guideline. If you have low PSI at lower RPMs that goes up at higher RPMs, it is an indication the main or rod bearing have been wearing and are no longer able to hold back the oil sent to them like they once could.
Of course, there could be other causes of low oil pressure. A friend of ours has a 1967 Road Runner with a very built 383 engine, to include a nice roller cam and roller hydraulic lifter kit. Recently he was romping around a bit, and suddenly his oil pressure dropped to almost 0 PSI, and at the same time the engine began to run very poorly. It turns out on the driver side of the engine the bolts that hold down the rocker shaft was not torqued (he could not find a socket drive adapter so he torques them "by hand", and the bolts all backed out and let the rocker shaft get pushed out of position. When that happened one pair of lifters popped out of their bores, so the oil that was sent into the lifter bores no longer had any restriction on their oil flow, and the overall pressure dropped due a large leak having been introduced.
Anyway, I have often found a lot of low oil pressure readings are caused by sending units that need to be replaced. Somethings the oil in their Bourdon Tube area has begun to leak by the seals in the sending unit. Other times oil has begun to leak inside the workings of the sending unit, where the circuitry is contaminated by oil, resulting in inaccurate readings. I hear of one case where some debris somehow got into the oil channels and became lodged inside the sending unit test port, which kept the sending unit from seeing the actual oil pressure level (I am assuming a high pressure situation caused the oil pressure relief valve to open, which sends unfiltered oil inside the engine).
I found what looks to be fairly useful ways to test an oil pressure sending unit and its electrical system in my cursory look at the shop manuals I have. I pulled out some excerpts that may be useful. One thing to check for is to see if adequate voltage is present at the oil pressure sending unit. That file with the excerpts I felt would be helpful is attached to this post.
