The only mechanical pencil lead I've found that matches the smoothness of a good rollerball or fountain pen is Pilot Neox in 4B. Although it's my favorite pencil lead, it has the significant disadvantage of smudging easily.

https://www.amazon.ca/Mars-STAEDTLER-Technical-Pencil-Set/dp...

They've got the pros of both mechanical and standard pencils without the flaws of both. I use one for writing and sketching and i've had the same pencil for 12 years now and still use it to this day. The refills can be a bit tricky to find though in stores. Art supply stores sometimes have them and i've seen them at places like staples.

When I was in high school, I bought a .9mm mechanical pencil, and it was glorious for standardized tests. Filling in circles was just a quick spiral inward, and the forms with rectangles were even easier — just a swipe across. I still have the thing, even though I haven't taken a multiple choice test in over a decade.