@ArduinoM: Be very careful about using any sort of tape in an oil tank. If the oil causes it to partly break down, or breaks down the adhesive, it will be a real b*tch to clean scraps out of the tank. Even worse if scraps get sucked down the outlet pipe.
Also while that construction method would create a capacitor, the oil would be on the OUTSIDE of the capacitor. It needs to be between the two electrodes in order to substantially affect the capacitance.
Ultrasound is probably easiest. Pressure is the most common method for industrial instrumentation, requires access to the bottom of the tank. Is this an above-ground tank or buried?