I had this problem for a while in my earlier setup. I was not able to get the skimmer to pull from the surface, which was the obvious solution. I tried to use a overflow cup mounted just below the surface for the water to fall into, then into the skimmer....but as the water level in the tank wasn't dead stable, the overflow cup would either not work, or run dry.
So....what I did was to route the skimmer return so that it FALLS into the tank, like a waterfall. As it does this, it breaks the surface tension and pulls the oil slick down. It worked OK.
So try to pull the water from the surface first. If that doesn't work try the waterfall.