Are Shimano Hollowtech II bottom bracket spacers unique? I'm moving my Shimano Hollowtech II XT crankset and bottom bracket over from a bike with 73mm bottom bracket to a 68mm bike. The only problem is that I only have 1 spacer and now I need 3 bottom bracket spacers.  Do Shimano external bottom brackets have a unique spacer or can I just use any bottom bracket spacers of the correct diameter and thickness?
This link implies there's no difference, and looking at what the spacers do I agree. Since they're a purely passive component that just fills up space, it's only the physical size that counts. Even with an extra 5mm of spacer I suspect you'll be ok, but I would be somewhat concerned if you had to put all the spacers on one side to maintain chain alignment.

BTW, this pdf and this page match my experience of those BBs. They're not great.