When your car is running slowly and also when it comes to a stop, the engines is near or at idle speed. That is often around 600 to 900 RPM.

An engine's oil pressure depends upon several factors including the engine temperature, oil temperature, oil viscosity, and the amount of  engine bearing wear and other tolerances. Also, if there is an inadequate amount of oil present, oil pressure can be intermittent and low.

If any of these parameters is in the not good category, oil pressure may decline.

The oil pump operates directly off the engine and will pump oil at higher pressures (speeds) as the engine runs faster.

Therefore, as the engine slows down approaching idle speed, the oil pressure will naturally and correctly decline. This is normal.

You can affect the oil pressure situation by being certain to:

--  Use only the oil viscosity recommended by the manufacturer;

--  Ensure that there is always adequate oil in the engine;

--  Change oil at the recommended intervals to minimize engine wear over the long-run.

Another factor that may impact whether or not the oil light comes on is the accuracy of the oil pressure sensor. If the sensor is not working properly, it may send a low signal when the oil pressure is not actually low.

Also, if the engine's idle is dropping lower than it is supposed to, oil pressure may decline to a point where the pressure sensor is validly warning of pressure that is too low at that speed.