In early tests I used vegetable oil as the heat transfer fluid. The temperatures used for testing are very close to those used in a deep fryer. Vegetable oil is also easier to handle then motor oil or other oils. Most oils are stable around 200-250C. Above these temperatures most oils release flammable vapours and must be pressurized so they are not boiled off. I wanted to avoid this complexity and danger in this system so I went with an atmospheric oil loop that operates below 250C.
The oil is only used as a heat transfer median and is not burned in anyway. Most oil can be heated and cooled hundreds of times before they need to be replaced. Several solar thermal systems use vegetable oil as a heat transfer median and only replace the oil every few years.
In order to increase efficiency of the steam turbine the system temperature may need to be increased (see design choice 4). If this is the case a pressurized oil loop would be required or an oil would need to be used with a higher unpressurized operating temperature.
Possible oil improvements include an oil with a higher heat capacity, or lower density to save weight.