Furnaces (gas or oil) should be inspected by a licensed contractor annually, and replaced when recommended. 

We also recommend annual cleaning of your oil or gas furnace:

  • Before performing any maintenance, be sure to switch off the electrical power and fuel supply
  • Clean the combustion chamber. Soot tends to build up on the chamber walls and can cause corrosion. Remove any loose material with an industrial shop vacuum and inspect the chambers for corrosion before replacing the cover.
  • Next you should inspect the exhaust flue. Check for holes, paying special attention to the areas where the pipe meets the furnace. Patch any holes with foil tape. Unfortunately, if your pipe has begun to corrode it should be replaced.
  • Check the oil filter and air filters and replace them when necessary. 
  • Adjust the burner to maximize efficiency
  • Finally, keep in mind, that you should routinely be checking and cleaning your floor vents. 

Source: https://www.thisoldhouse.com/how-to/how-to-do-fall-furnace-maintenance