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.