I had one 2 oil furnaces in my houses over the years. The latest house with it, I was very diligent caring for it. And, whenever the service tech would come out, I would glean as much as I could from him. Invariably, they would tell my to open the small inspection port during the full burning operation (not just start up) of the unit and it must be burning bright orange. Otherwise, it is out of adjustment and needs servicing----I did learn how to do some nozzle replacement and setting the electrodes and some burner adjusting…but I left the annual stuff to them as that was their business.