Is there an all-natural, non-volatile and low viscosity oil? The goal is to thin a vegetable oil so that it spreads easily on top of water. It is not for consumption and the flash point of the mixture should be >=95° Celsius". Vegetable oils tend to have flash points greater than 225° Celsius so the flash point of any additives can be well below 95°, as long as special precautions would not be necessary in a manufacturing process.

Since my original post, I've discovered another reason for the low viscosity. It is required for the delivery system to work properly.

I am open to other means to achieve the goal.
If the goal is to have the oil cover the water surface, then you might need to add a natural surfactant, such as lecithin, to cause the oil to spread, rather than form globules. You can try egg-yolk or soy beans as a source of lecithin, or you can buy refined lecithin.

As for the smoke point, or maximum cooking temperature for the oil, many common vegetable oils could do: safflower, sunflower or refined canola (rape seed) oil all have a good margin of safety. 

One issue might be that if the water is at a "roiling" boil, the turbulence will mix the oil into the body of water. For that reason, you'd need to keep the temperature below 100° C and prevent local hot-spots in the heat source from causing strong convection currents.