Can I use a PC3-10600 memory on a MacBook with PC3-8500 slots?
You should be able to use the faster RAM without a problem.  No guarantees when it comes to RAM compatibility that hasn't been vendor qualified (which is seldom an option) so make sure you buy from someone with a good return policy.

You might consider buying through somebody like OWC (macsales.com) that specializes in Mac parts.