I have used 3 different systems to heat my oil so far...
1- I used a O/A torch like Ed Fowler does in his video....It used up too much of my gas so I stopped doing this.
2- I used some bricks to support my quench oil tank above an electric single-burner type hot plate, works like a charm even in the -20 below zero of my shop, its slow but its cheap.
3- When in a hurry, I just decide to fire up my forge and spend a bit of time hammering on the next blade as a few sections of left-over steel heat up inside the forge untill red hot. I then just lower them into the cold oil, fight for my life a bit with the fire that is the result of this foolish system, and then lower the next section of steel until I get the oil to the right temp....this is a fast way to heat my quench tank...gets my heart a pumping too.