SchrodingersCat,
Welcome to the forum and thank you for your question. The operation of different types of oil skimmers are similar. For disk, drum, and mop skimmers, Oleophilic material in the shape of a disk, drum or mop is placed in the oil contaminated water. Oleophilic means to have an afinity with oils...oil will "stick" to the material, while water will run off. If you can get some of this material, you could potentially attach a disk and/or drum to a RC model engine to rotate it in the contaminated fluid.
The issue is that the efficiency with which any of the mechanical options work is highly dependent on the viscosity of the oil and the depth of water they must work in, and sea states (rough or calm water). Viscosity describes how well a fluid will flow. A low viscosity flows freely (like water) where as a highly viscus fluid will not (like molasses). Keep in mind that fluid viscosity can change with conditions, such as temperature.
Skimmer devices tend to be designed to work well for certain conditions...i.e. some work better for low viscosity oil spills, while others work better for high viscosity oil, like crude.
My research suggests that the skimmer devices, if operating within their design parameters, have efficiency ratings around 90%. I would suggest, rather than spend time trying to "engineer" homemade versions of highly industrial equipment, you vary the viscosity (thickness) of the oil contamination and plot the efficiency of the ferrofluid over the range of contaminaton viscosity. Any point you are above 90% efficient, you will be better than the skimmer designed to operate at that viscosity.
This link includes a plot showing skimmer types efficiency vs viscosity.
http://www.oilspillsolutions.org/skimmers.htm