@AI: It is my opinion that pencil grade is irrelevant for all of these throwaway exercises. They are not about the marks they leave on the paper but about the experience and knowledge gain happening in the mind outside of verbalization and technical details, about building muscle memory and training the mental model of things, observation skills, creativity improvement, living through the performance, not about leaving graphite trails or any other markings on the paper. For artworks, that's different, there material matters.
Either way, I don't know as for the throwaway exercises I'm using up a bunch of old pencils that just accumulated over the past 25 years or so from all kinds of places. Giveaways from hardware stores, ownerless pencils found on the street, leftovers from pencil boxes. They are all kinds of different grades, Bs, HBs, 3Bs, 2Bs, some are of really grainy/crappy/broken mine quality and not even marked. Also not using the same one each day, just grabbing any random one, usually one that I don't need to sharpen, so that I can just start drawing.
If there's a fluctuation in appearance, it's likely due to fluctuating mood, coffee level, random pencils, varying scan settings, different pressure and or speed while drawing or anything else but again, that's all irrelevant for the throw-away exercises.
Also yeah, if it wasn't for body deterioration coming with age, practicing for 100 years and still learning new things every day is well possible. I have slowed down a bit, been on schedule 7 for two weeks already (only did A,B,C so far minus the 5h long study). 3h a day was killing my back/neck.
