If you point one of your powerheads towards the surface you should be able to create enough turbulence that the oil slick will fix itself, just keep the powerhead permanently in that position.
I also think you have a case of the microbubbles as Zoa Elite suggested.
Microbubbles can be difficult to distinguish from particulate matter if you don't know what you're looking for. You say your skimmer is a fair way from the pump and lower than the pump...are there any baffles in your sump? Could you post a picture of your sump? Also, if you have an external pump, and there are any leaks in the plumbing between the sump and the pump it will suck little bits of air in, chop it up, and blow those into the display...which sucks.