Thank you for asking for clarification. Comfrey leaf/stem and root can all be used interchangeably it’s just that the action is much stronger in the root, but the leaf/stem make a wonderful oil as well. So to answer your question, I’m not sure which part of the plant David would have been expecting in the oil, but either root or stem & leaf will make wonderful, potent medicine.
I tend to make it with root, if I have had time t dry out my roots sufficiently to feel comfortable infusing them into oil. They are so so juicy with all of that beautiful mucilaginous goo, that I tend to use the root for blending fresh into a poultice and the leaves (wilted) to infuse in oil.