I have been noticing that occasionally the oil pressure light comes on. It happens in low rev situations. It always flickers out. In the summer, I would notice that the light would stay on a little more than normal (about 3 seconds) upon startup. Now that cold weather has come on, I have been noticing that it stays on even longer upon startup when it does come on, and occasionally it comes while I am at a stop light or parking lot. Again, it does not occur often. What could it be?