I use Rislone oil treatment. It gives green exhaust for the first second or two after starting.

A tangential story. I was flying my Dmeco Sportwing with ST G20-15 in control line Old Time Stunt at a Sig control line contest. I foolishly set my fuel can sit out in the hot sun. I could not get a long enough run to finish the pattern. I was using one of the Sullivan black bulbs for filling. I filled it with fuel, clamped the tube off and put it on ice. With the cool fuel I finished the pattern with no problem.