2018 Denali xl oil gauge drops from 40 to 20 when it is idling.
First, check to see that the oil level is correct; low level will do this. Otherwise it's most likely the oil pump and/or pressure relief valve, or could be worn-out crankshaft and bearings. The oil pump runs at a fixed pressure and the relief valve opens to reduce it to operating pressure, then the bearings create pressure resistance. The Oil light is just alerting you that you have something seriously out of acceptable operating range. A more remote possibility is that the oil sending unit is bad.
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