Sounds like worn bearing clearances are causing the low oil pressure at
low RPM. I had the same thing on my old '91
Grand voyager that had accumulated 350,000 miles when my wife destroyed
by driving it with a coolant leak. I ran 20W-50 oil in it
to maintain oil pressure at warm idle. The problem is that heavier oil
is a 2-edged sword. It keeps oil pressure up when warm, but during
cold start-up it's thick and doesn't flow as rapidly just when you need
it the most, to reduce wear during cold start-ups. So you actually wear
out the engine (and thus the bearings) faster this way.
First things first. Have the oil pressure checked. More than one oil
pressure sensor has failed on those vehicles, and more than one engine
has been torn down due to a "percieved" oil pressure problem.
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