I am not sure this would work, but adding -lamp black- to the oil in a ratio to keep it suspended might work. I am not exactly sure where we got this for another purpose several years ago. Lamp black is just carbon powder as a result of combustion. Very dark black. Should disburse nicely and not cause environmental damage.

The dye ideas really are not good around people. There may be other powders that could be mixed with the oil... of course it would have to be mixed prior to flight to keep from settling.

If you try the idea, could you let me know if it worked?