I have a elos tank with the same setup. All you have to do is pout the elbow return to have a 45 degree angle down. This will stop the flow that is pushing the oil slick away from the overflow.
I didn't read his post clearly. Your picture shows it on a downward 45 degree angle. I read straight down for some reason. Regardless if you can't get it to work properly I would go the route audiofile18 has gone, since he has dealt with the same problem.
Lower the durso pipe in the overflow. Happened to me when i first set my tank up and could not figure out why it was happening. Pointed the powerhead at the surface and did nothing. Lowered my durso so the water had to fall lower then the teeth like 3" lower then the teeth and in 4 days it was gone