In this case you may consider to use two or more heat sensors. One placed close to the heating element to prevent overheating of the oil in that area (which may be a concern - this sensor would stop the heating when the oil becomes dangerously hot here, of course this temperature will be higher than the bulk), another (or even multiple) at the opposite side of the the tank to tell you whether the overall temperature of your oil is high enough.
For heat sensors: I'm thinking of thermistors or thermocouples, as these are very robust and have high chemical resistance. A properly encased DS18B20 may work as well, should be easier to read out.
It sounds like it's a pretty big tank - what sizes and power and so are you talking about?