Most rotary evaporators come with baths that go well above the boiling point of water - usually up to 160 or 180°C. That raises the question: what do you use in the bath if you want to achieve a temperature above the boiling point of water?
In reality, water only is functionally useful up to about 80°C or so, and perhaps even less for long processes, as the rate of evaporation becomes too high as you approach higher temperatures. Beyond that point you will want to use a high-temperature bath liquid. The most common is silicone oil. Silicone oil has a useful working range in baths from about 25°C up to as high as 300°C depending on the specific silicone oil you get (always check the manufacturer specifications to determine the working range of your particular oil). Always ensure the temperature of the bath is below the flash point of the oil.
Silicone oil will turn brown and degrade over time. Ensure you check the color of your oil regularly: if it's getting darker, that means it's time to replace it. It can also make your glassware slippery so take extra caution when handling glassware that has contacted silicone oil.
Various synthetic thermal liquids are also available with a wide range of properties.
SAFETY NOTE: Be extremely careful and follow proper laboratory safety precautions when using high temperature baths. Always wear proper protective equipment, ensure that high temperature vapors are properly contained, and avoid splatter of high temperature liquids. Appropriate shatter-resistant glassware should be used for high temperature applications.
Julabo high-temperature bath fluids
UPDATE (6/29/2016): We've ran into a few customers using vegetable oils in their rotary evaporator baths who were having issues with bath heating. Using a vegetable oil is not advisable as it has a much higher specific heat and viscosity than silicone oil or other synthetic oils made specifically for heating baths. Not only will vegetable oils take significantly longer to heat, but they may potentially damage or decrease the life of the bath due to an increased load and overheating of internal components. While we never condone using vegetable oil in a heating bath, we do know that because cooking oils are much less expensive some customers will choose to use them. If you insist on using a cooking oil, which we do not recommend, corn oil is probably the best due to its combination of relatively low viscosity and specific heat compared to other cooking oils.