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Those pressures are close to what I see on my son's car (300,000+mi), using 5W30 Mobil-1 synthetic. It shows ~52psi at 2000-2500RPM and between 25-30psi at a 700RPM idle, with oil at full operating temp on a warm day. Bringing RPM's up even the slightest over idle speed gives a quick jump in pressure.
I removed the oil pressure gauge (VDO "Cockpit") from the 83's dash a few years ago at about 150k miles, as the nylon tubing to it sprang a leak and it wasn't really telling me anything new...consistent readings over 10+ years and about 100k mi. At the time it showed pressures very similar to my son's 81 with the same oil and similar conditions, maybe 2-3psi higher.
Differences of 2-3 psi are starting to get into issues of gauge accuracy and one's ability to judge exactly where the needle is!
Given your hot idling pressure of more than 20psi, I think there is a malfunction in your warning light sender. They are designed to turn the light on at something like 7 or 8psi. If the only time your light comes on is at slow, hot idle, it's not likely there is anything wrong with the light wiring.
Switching to a higher viscosity oil is unlikely to accomplish anything except increasing engine wear at cold starts.
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Bob: son's 81-GL, dtr's '94-940, my 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.
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