Oil filled radiators are fitted with a heating element and filled with thermodynamic fluid. As the element warms up, the liquid expands and rises to the top of the radiator, creating a convection cycle. Their excellent thermal retention means they emit a third of their heat as radiation, which results in long-lasting warmth.

Usually a silicone oil (mostly siloxanes like polydimethylsiloxane). theory is the oil doesnt get 'hot' just warm enough to flow freely through convection, most siloxanes hit B.P between 150-350°c. A safety margin is always a good call, general 'guideline';is between 10/15% of the cavity volume left for expansion.