Siegler Corporation transitioned to making electronics during the 1950s, so their oil heaters have the disadvantages of heaters made at the time. As a result, their efficiency pales in comparison to newer units.
Heaters made in the mid-20th century generally used efficient combustion processes. In addition to producing less heat, inefficient combustion also creates more potentially dangerous by-products. Insufficient ventilation can make older units dangerous to use.
Older furnaces and boilers also generally use insulation far less effective than newer insulation, which causes heat to bleed into areas where it isn't needed. Those who want to upgrade an old heating system may wish to insulate their pipes and other components. Switching to a newer model, however, can result in substantial savings.