Silicone is flammable -- I've set silicone rubber on fire with a blowtorch a couple times, and it's extremely hard to extinguish -- but the temperature required to start combustion is so high that it's irrelevant. In a RC context, nothing short of a large lithium battery fire will cause silicone to catch fire. Silicone oil might actually evaporate before it has a chance to combust, whereas silicone rubber at least has the decency to hold still while you blowtorch it.