Generally available fluid with good heat transport properties
400 C is beyond long term use of any organic heat transfer fluid.  Dow Corning boasts 427 C for one of its silicone fluids.  Don't bet on that.  Inorganic eutectic salts will long term decompose into corrosive volatiles.  Best bet for a red hot closed system is fusible alloys of indium, gallium, and bismuth - beware of metal boiling points.  Open systems will air oxidize.   Accommodate sealed inert gas fill (argon) with a metal bellows add-on, preferably on a pipe extension away from the hot zone.  Fill composition must be compatible with container composition.  Mercury is out for toxicity and boiling point, 357 C.  Thermal coefficient of linear expansion, solid vs. liquid alloy, must be looked up.  The increment is cubed for volume change.

500 C (red heat) will change the metallurgy of stainless steels over time.  They will harden (carbide precipitation), among other things.  Don't try to drill a hole or tap a thread in stainless that has been soaked at red heat.