According to the invention, a silicon oil can be used in a thermostat, said silicon oil being added in small amounts to a stabiliser. The stabiliser is, for example, a pyrogenic titanium dioxide and binds potential reaction partners or reaction products in the silicon oil. As a result, age and temperature related alterations of the silicon oil can be reduced, and the heat stability thereof can be increased. A precisely justified and constantly functioning thermostat can be produced over a period of time.