OK, I have found that I also had oily residue on my sump tank. The main tank was recently containing oily residue and no matter how much surface agitation.... Still oily.. Today I decided to go a little further than just the weekly water change.. I changed the filter sock and instantly the oily surface was GONE.. I therefore can only recommend that you maintain a strict habit of cleaning your filters and whatever keeps your water clean and I am positive that your oily residue will be a thing of the past.. I have two tanks and I find that the tank I have at work, that does not get looked after as carefully as the one at home is the one that starts to have issues..
Tank - 1 - OFFICE
Aquascape - initial setup March 2014 and then rescape August 2014
CADE Tank 800 x 800 x 600 - 320 litre
Sump filtration - Long noodles, 3 Layers Filter Pads. Carbon
UVIEW sterilizer
CO2 with inline reactor
ADA fertilising
ADA soil setup
RAZOR LED lighting
Twinstar
Tank - 2 - HOME
Juwel 120 litre tank - setup Dec - 2010
Fluval 406, cannister
Inline reactor c02, compressed
LED lighting