There is already oil (caster or synthetic) in the fuel. Run it a little rich and you will see a smoke trail.

If you are looking a smoke system they generally have a seperate tank for smoke oils and they Don't burn it through the engine they use the heat in the muffler to generate smoke instead.

BTW the engines you list don't burn "gas" they use an alcohol based fuel with different amounts of castor oil and or sythnetic lubricants and nitro-methene is also added to most sport fuels the engines you mentioned will run fine on fuel with 10%-15% nitro in them.