Why might we have an oily smell coming from our vents in the car especially at stoplights?

I think that you may have an oil leak,which is getting on to the front section of the exhaust and whilst travelling along the fumes are being blown away, unlike standing at trafic lights, they are rising up under the bonnet and drifting through the heater/air vents, especially after along motorway run ,I have also had this happen to me!!