90 Bonneville low oil pressure problem

About a month ago the oil pressure gage in our 90 Bonneville SSE started a trend downward from the normal 3/4 mark. Within a couple of days, it wouldn't go
above the 1/4 mark, and at engine speeds below 1500 RPMs it is in the red and the oil pressure light comes on. When warm and at idle, the needle doesn't move off the bottom at all.
I first suspected the sending unit, replaced it with no change. So, I took the front cover off and inspected all of the oil pump parts to the specs in the Haynes manual, no real wear, no problems found. I also dropped the oil pan and confirmed the pickup wasn't blocked and was intact. I didn't remove any bearing caps but I tried wiggling each rod, nothing seemed amiss. No metal particles in the the oil or pan. So, I suspected the gage circuit and put it back together - after all, if the oil pressure was that low, the fuel pump would have kicked off. I finally got a mechanical gage tied into it last week and to my horror confirmed the dash gage was reading properly. I picked up an oil pump repair kit and a used front cover from an 89, but the 89 didn't have the cam button boss on it and was in much worse shape in the oil pump area, so I put the kit in my old cover. This included the relief valve, and I did put in the same type poppet that was originally in it (without the holes). It's acting the same.
The engine doesn't make any unusual noises. The lifters don't clatter, either during startup or running (although they did clatter for a couple of seconds when I first started it up after the dissassemblies - normal oil change dry start-up stiff). I'm concluding the oil is getting to all of the parts of the engine, but just not under proper pressure.
I'm suspecting an internal oil passage has opened up, probably towards the end of the system, but I'm not familiar with the 3800 oiling system. I did verify the galley plugs behind the timing gear were in place.
For reference, the car has 196k miles on it, we got it from the 2nd owner with 104k miles in 2000, the first 6k miles of its life were as a California rental car. I've had AMSOIL in it the whole time we've had it. Other than crank sensor and coil pack failures, this is the only engine problem we've had with it.
Any and all help & suggestions would be greatly appreciated. We're not ready to give up on the car, but an engine change is not economically wise. I have the ability to remove, disassemble and reassemble the engine - in fact, I've been driving this car to work because I'm replacing the 305 in my '82 Camaro with a 350 I built up. I rebuilt my first engine in 1972. I just need some info on this 3800 before I start tearing things apart.
Thanks in advance.
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five7kid
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five7kid wrote:

I would have used a new front cover as it's really an integral part of the oil pump. Having said that, it sounds like you may have excessive camshaft bearing wear. This will give you the type of low oil pressure you are describing. You won't really know though unless you tear the engine down and inspect the cam bearings and it wouldn't hurt to plastiguage the mains and rods at the same time.
Ian
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sounds like you spun a cam bearing or two
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