I would suggest you stay with the 10mm one like the manual says, be sure you don’t over tight it.

I broke my gear box when I put 15mm on it, I’m guessing more thread also weakens the gear box. Another suggestion, I put 1mm shim below the chassis brace (on the side of the gear box), I noticed chassis brace bends a little bit when you tight the screw if you don't use the shim.