Alas, I have journals from the 80s in pencil that are already nearly unable
to be read. Mechanical pencils probably help a good deal - I can easily tell
when the pencil was sharp because the text is more legible.
As the line gets wider, it becomes faded quicker over time. It's not
particularly not-sharp, just the normal "used for awhile" pencil. Even the
sharp parts has faded as well, just not as profoundly.