Acura Mechanical Pencil Lead 2B 0.5mm GHQ688

With modern leads, 0.5mm is quite strong enough for most people, but more heavy-handed people do sometimes find they snap such thin leads, and 0.7mm or even 0.9mm leads are better for them.

A good lead can be expected to last for decades, if you don’t abuse it. The simpler the mechanism, the less there is to break or malfunction. My oldest are twist mechanism Eversharp 1.1mm pencils.