The light is telling you the oil PRESSURE is getting low, not the oil level. Make sure your using the correct weight oil. I've seen this happen when too light an oil is used, like 5-30 instead of 10-30, the oil thins out at temperature and the pressure goes low at idle. Check the oil pressure with a mechanical gauge, low pressure is nothing to mess around with.
I've also seen this happen when there's crud built up at the sending unit, so you might want to check that also.
- Joe