Put a real gauge on it- - - -at least for diagnosis- - - -don't trust the dash gauge to be more than a wild-donkey guess about what's going on, especially at hot idle. The idiot light on a 327 and most other small block V8s turned on at 7 PSI or less. That's one reason the factory stopped putting gauges on vehicles in the late 1950's- - - -they really didn't want people to see how low the pressure was going.
Small block Chevys with some mileage on them tend to wear out camshaft bearings, which causes a pretty good pressure loss at hot idle. Unfortunately, it takes a full engine teardown to a bare block to swap cam bearings. If you drop the oil pan and install a high volume oil pump you can avoid the appearance of zero or very low pressure at idle- - - -but it's like taking an aspirin for the headache caused by a brain tumor- - - -it's treating the symptom ,not the real cause of the problem.
Jerry
