If you have fuel in the oil tank, there has to a leak. If the engine has an oil cooler that uses fuel as the coolant, the fuel is at a much higher pressure than the oil and will flow through the leak into the oil system. You can land with more fluid in the oil tank then you departed with. If you have oil quanity gauges for each engine, you might think about shutting the engine down since Jet A does not have the same lubricating properties as oil.