Chris is correct..... 400 degrees, is 400 degrees, whether that be from a liquid, or air (liquid being conduction, air being convection) The key is..... will the device offer the accuracy required to heat and HOLD the oil temp for the duration needed? I have no doubt what you're wanting to do is possible, but you're going to have to do a lot of experimenting, sizing the oil tank to make it work well. It's not totally about the accuracy of the heating device...... you also have to take into account the "thermal mass".....which includes the amount of oil, and the container holding it....... and the amount of "swing" that takes place when the device turns on/off trying to maintain the set temp. When it comes to tempering.....anything more then a 25F degree swing is unacceptable to me.
My experience with this comes from high and low temp salt tanks...... very often people use high temp salt tanks for austinzing because of their ability to be extremely accurate..... but there is a catch. The amount of salt equals thermal mass. Or in your case, the oil would be the thermal mass..... the least amount of thermal mass you can use to accomplish the job, the better. Why? The more thermal mass, the more "swing" you will usually have... that means the more thermal mass, the more you will undershoot, or overshoot the set temperature. The way I learned this was trying to help another individual with building a salt tank setup...... My tank is 2 1/2" ID X 18" long..... just big enough to do the largest knives I generally produce. When helping the individual I mentioned, he insisted on using a 6" ID X 24" long tank..... he stated he'd likely never need that big a tank...but he wanted it. I did my best to try to explain that he was gona have issues with accuracy..... and he did. His salt tank setup would hold to only within about 200F of the set point..... what happens is that the thermal mass was always trying to play catch up..... the salts would get to temp, the burner shut off, and the temp continue to climb for about 100F.....then it would start to fall until it reached the point where the burner was set to kick on.....and the salts would continue to fall for about 100F before it started rising again........THERMAL MASS.
Same holds true for the "low temp" tempering salts..... folks usually use a product called nitre blue from Brownells for this purpose. These "salts" melt at 400F.....and have a working range up to 900F. The "high temp salts" I mentioned above, melt at 900F, and have a work range up to 2,000F.
I believe the reason we don't see more "liquid" tempering, is because the same results are much more easily and safely achieved with air.....in either a heat treat oven, or even an accurate kitchen oven.