Q: Oil pressure light turns on at idle

First, make sure you are using the right viscosity engine oil, as specified in the your vehicle’s owner’s manual, and install a new oil filter using only a dealer-supplied original equipment manufacturer (OEM) filter. If after executing those two steps the light still comes on, there only two physical possibilities, one of which is highly improbable: 1) only at idle, as pure coincidence, the circuit to the warning light completes (improbable); or 2) at idle, the oil pressure is, in fact, below the threshold that the sensor is set to detect. To confirm exactly what is happening at idle, a certified technician will "T" in an actual gauge and set it within the passenger compartment for convenient viewing. Each and every time that the warning light comes on, you will then be able to record the pressure (that is, the exact physical measurement). If the pressure is below the specifications found in the factory service manual, then that is why the warning light is coming on. If the pressure is above the specifications, then there is something wrong with the sensor and/or the circuit, but most likely it is the sensor, particularly if it is not an OEM sensor. Consider having a certified technician, such as one from YourMechanic, perform this diagnostic for you.