Hello,
I would not react to hastily. The oil pressure at idle will depend on idle rpm, coolant temperature and driving conditions.
It is normal for the oil pressure to fall off the peg after the engine is fully warmed 179 degrees F and at a normal idle of 700rpms. Depending on the condition of the engine, the driving conditions the type and condition of the oil, you might see more or less radical fluctuations in your oil pressure at idle.
There are two oil pressure relief valves in the 280 series engine to prevent excessively high oil pressure. One screws into the front of the block (under a plug next to the timing pointer). The other is on the oil pump itself. It is very unlikely that both of these are stuck. It is more likely that your gauge has become inacurate or your oil is not being warmed enough to make a differenc.
Check the oil presssure after a brisk highway drive then an idle at 700rpm, make sure the engine temperature has risen to around 179 degrees for a period of time.
Joe Alexander
Blacklick, Ohio