Actually oil burning fireboxes are only minimally lined with refractory/fire brick. The refractory's primary purpose is to retain heat that is continuously reflected back into the fire to aid combustion. The fire pan which is below the mud ring is lined for this reason as well as protecting the steel as it is not water backed. The side sheets are typically refractory covered up only a few courses of brick and the door sheet somewhat higher to provide a flash wall depending on the engine. The fire blasts against the refractory on the door sheet which is very important in aiding combustion due to the heat reflected back into the burning oil/air mixture. It is a trade off covering the water backed areas with refractory as it would tend to reduce heat transmission in those areas, vs enough refractory/brick to reflect heat and aid combustion.