I've had great luck with capacitive sensing and water levels.. In my case I was using the Teensy 3 which has built-in support for cap sensing that seems more reliable than when using the arduino cap sense library.

For the sensor, I got a sheet of copper foil with non-conductive adhesive on one side (digikey sells this). Using scissors I cut a 1" wide strip several inches long, with a couple small sections where I removed a piece 3/4 as wide as the strip (sorry.. hard to describe) The end result is sort of "F" shaped (where the horizontal bars are thick and the spaces between are thin).

For shielding against outside interference, I put another rectagular piece over the whole thing, attached to a ground lead.

Stick the whole thing to your oil tank, and you should see large changes in the cap sense value as soon as the liquid level crosses a gap.

here's a web page with something similar

http://soulysolar.mudventions.com/?p=681

hope this helps
David