Hi. My RES produces her own oil slick when she's been eating salmon or other fatty meats (yes, she's a 19yr old RES & still a full fledged carnivore - but then that's a story for another message thread...). Anyhoo, we often find that there's an oily layer or even white blobs of grease/fat after she's eaten - both as a result of oil on the meat's surface and from stuff she's pooped out. Our solution, like that of Turtlemoon, was to feed her in a secondary tank/tub (actually it's just a giant tupperware container that we can leave in the bathroom or put out on the porch so she can bask after feeding). We also try to keep her in that until she's pooped in order to minimise the oil slicks in her big tank. Don't know if this applies to your turtle, but hopefully it'll help you solve the mystery...