While oil can be heated up to a higher temperature, you're adding a lot more volume of material to be heated, which in turn will take longer with the same amount of BTU input.
I can only talk about our equipment as ours are typically ran as a pressure vessel. We suggest using water on our systems, although you can also use oil without pressure, and here is why:
1. Water is more cost effective than oil, requires less, and is 100% easier to clean up.
2. Not sure how your kettles are designed, but ours have a reservoir on the bottom that houses the elements. Where you need to fill the jacket up exponentially with oil, you only need enough water to keep the elements covered while generating enough steam for the jacket. When under pressure at close to 15psi steam is almost 250F.
3. While you can hold higher temps with oil, you can get the steam temp up much quicker and cool it much quicker, making it a more responsive system. In a well insulated kettle, once you get the oil up to temp, it takes a while to back that temp down.
That's based on our designs, our research, and feedback from our customers.