We use 70# paper on all of our products. It feels so good to the touch. Pencil, traditional ballpoint pen ink, gel pens, Flair medium & fine tip felt markers, and highlighters have been tested and have minimal to no bleed-though with normal use.
However, we recognize that some products have more saturated ink. For instance, in testing, Sharpie markers have moderate bleed-through.
Additionally, the user could be heavy-handed with markers, which could create bleed-through.
We always recommend testing markers and ink in an inconspicuous area before use.