No problem, we’re not talking gospel truths here. 
As for other types of string, I use slick 8 50/50 on a lot of my fixed axle yoyos. Works about as well as cotton, and lasts way longer.
Wow, fixed axel play goes a lot deeper than I thought. Does the wooden axel add any weight to the yoyo. The butterfly is very light.
Also, how do you get the metal axle out of the yoyo?
The wood axle won’t appreciably change the weight.
You remove the axle by carefully twisting the halves apart. Generally the axle will remain in one half. Use a pliers to pull it out. You can also pull the halves apart by using a screw driver to gradually pry them apart, moving around the edges. Pry a bit, move, pry a bit. repeat until it’s loose.
Some people use a 1/4 inch dowel. The metal axle is thinner than that. You can also get thinner dowels. You may need to enlarge the hole to get the axle to fit.
Do you need to shave off some of it all around it to make it fit into the hole?
Never had any trouble fitting the 1/4" dowel in the Butterflys with the old, smaller axle. It fits tight but you don’t have to force it in or anything. The new axles are actually a little larger and the axle hole is a little bigger. For the new axles there is actually a tiny gap at the axle hole with 1/4". The dowel fits perfectly in the indention on the cap. Just make sure you put your glue on the end of the dowel and that it goes all the way to the cap.
Here’s a pic of the new style axle (left) and the old style (right.)
It might be just the particular batches I got, but I found the type-8 I have isn’t wound as tightly as the type-10. It makes it play kinda weird. I love the way the type-10 plays.