wvmarle:
The DS1603L ultrasound sensor mounted to the bottom should work but temperature of the oil one may get in the way. It'll also need correction for oil, the standard output is for water.Or a pressure sensor attached to the bottom - if you have a tap there, that's easy. Get a differential one that can handle the liquids & temperature.
Is it the gravity non contact sensor?? There is no tap in the container 
Do you think the ultrasonic sensor HC-SR04 will be a good choice for this environment??