Raw Version: /g/3033731/50281b851d/

1-40: Greyscale
41-80: Screentone

I take no credit for the translation.
Translated by A Cool Person on https://www.dlsite.com/maniax/work/=/product_id/RJ01249057.html
Please purchase to support the artist and translator.

"1-40: Greyscale
41-80: Screentone"

They're both 24-bit RGB color format.
The only difference is, that page 1-40 are completely messed up in color tone. It's not greyscale.

You can convert page 41-80 from 24bit into 8-bit greyscale just fine without any quality loss, because it's black and white. I don't know how you managed to mess it up during the conversion. Either your settings are meh, or the tool you've used is garbage.
Edit: since the source is in JPG container, you've to set the output quality setting to the exact values as the source. You only want to change the color format from 24-bit to 8-bit without changing any of the other information in the container.
Edit2: I just did it. It saves only about 21% of the file size. Exact quality 1:1, meaning 0% data loss. I reduced the total size from 56MB down to 44MB. Not a big deal in this case, because the resolution is low so the total size. However, still the right thing to do imo. Keeping it in 24bit, or even 32bit (some retards even do 48bit) is just absolute waste of storage capacity.

Base +6, KuroToast +6, TheGershon +6, uberlawabiding +7