I'm not sure why the flash point matters in a closed system. There won't be any flash so long as the other gas in the closed system isn't oxygen. You could use argon, which is inert. Nitrogen might work too. The problem would then be mainly the chemical stability of the oil on its own, not whether its vapor would ignite.
There must be some engineering tables on the Web listing good high-temperature working fluids.
Mike W.
p.s. Of course, you will have to be careful about leaks. You don't want a small leak to the atmosphere to start a fire. There must be engineers who know more about this than we do.
(published on 07/02/2009)
