My thoughts are like wise; oil contamination via the cooling water pump is unlikely, there is a strict separation between both fluids. For the engine block we only have the main gallery casted in, and this has enough separation with the liner spaces. I would also point towards the LO cooler. I recommend to skim the oil form the expansion and maintenance water tanks in order to see the amount of leakage. The engine has been recently overhauled if I am not mistaken and I may be possible that oil spils in side cylinder heads, block, piping, etc. is causing the film of oil on the water.