While there would be numerous obstacles, one consideration would be tank capacity/weight. Oil is lighter than water, so the cars would not carry the same volume. Add the price of cleaning the tanks/plumbing etc, it might be better to scrap the tanks/plumbing and make new ones. They could be sized to lessen car weight for the Eel river section. You might have Potable and non potable cars. This would also answer people with "eew" concerns. On the plus side, cars could be dropped off at various "community" sidings, provide "weed water" etc. Some could be dumped into waterways that feed reservoirs. Nice thinking outside the box BTW