I noticed smoke one day coming from under the hood while driving! So, as I began cursing as I pulled over to see what the source was. I noticed oil had leak onto the driverside header and it seemed like there was some oil around the seal by the oil fill and Valve cover. I brought it to the dealership and they said they couldn't find a leak. They said it was old oil from an oil change! Meanwhile, I hadn't had an oil change in months. It never really smoked again but I still have the oild build up! What should I do?