A customer enquired as to which size S-Lon (Superlon) / C-Lon beading thread (AA or D) would work best with size 8/0 seed beads. Below is our response, which pretty much applies to seed beads in general, not just 8/0s.
Usually, we would recommend Size D thread with Miyuki and Toho 6/0, 8/0 and 11/0 beads, and Size AA thread with their 15/0 beads, but it partly depends on other factors:
1. If your project has mixed size beads, you'd want to make sure that the thread size will accommodate the smallest beads;
2. You will need to allow for the thickness of the needle, plus a doubled length of thread to pass through each bead. A thinner needle will allow for more passes;
3. What type of project it is: if you only need to go through 8/0 beads, say, 3 - 4 times, Size D should work; if you will need to make more passes through the beads it might be safest to go with AA. Size D will give your work more support, though;
4. What type of 8/0s you are using; Miyuki and Toho Japanese beads have quite large holes (we are unfamiliar with Matsuno, so not sure about those), and our Czech seed beads also have quite large holes. Other Czech, Chinese, Indian, etc. 8/0s may have fairly tiny holes.
In short, the most convenient thing is to have both sizes of thread on hand in case a last-minute switch is needed, but the above are fairly reliable guidelines.