An optical sensor would be a bad idea.That will go a long ways towards notice of any developing issues.
Do you have an oil level indicator? In the car business we found that temp sensors do not warn you when fluids are low, and will even lie to you about temps if the temp sensor is uncovered when fluid level is down. I like these - http://www.dwyer-inst.com/Product/Level/LevelSwitches/Optical/SeriesOLS - Wire it with a 2x22AWG shielded cable, hook it up to an input on the EFIS and set an alarm, and they run on 10-28 volts. Only drawback is the 1/2" NPT, and that is not awful. I am trying to figure out a place for one on a Lycoming...
From the 'one that I like':
3. Prism surface must be at least 2˝ from any reflective surfaces.
6. Inspect and clean lens at regular intervals. Level switch not recommended for use with liquids that leave deposits.
And this limitation:
Mounting Orientation: Horizontal or up to 45° from horizontal.
I like capacitive level sensors. Dead simple, nothing to clean