There's 2 different figures for oils as far as temperature goes. The first one is flash point which is what is usually talked about. Flash point is where the surface of the oil starts to evaporate fast enough to sustain a flame for an instant leaving a liquid still there. From memory, synthetics do this at around 430F or so while castor does it at maybe 450F, not all that much difference. But then there's the fire point which is when the oil is hot enough to be boiling completely and giving off vapours so fast it burns continuously. When you have a very thin film of oil it all disappears extremely quickly. Synthetics have a fire point maybe 10 or 20F above the flash point but castor is at 850F so basically it's impossible for any engine to get hot enough to burn off all the castor.