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.