I'd go with acetone as well, however; Beware of hardware-store acetone..
Especially ones from WalMart, they are loaded with stuff to hamper those who use their solvents for clandestine drug manufacture. Things that won't dissolve cleanly, etc. So i've read. It might not be a big deal at all because you aren't boiling the stuff down, you know?
The best lens cleaner (commercial, that is) that i've used is the Rexton Optyl-7 stuff. Note that this won't really remove things that are stubborn.
Just clean it nice and easily, not too rough. You knew that though, i'm sure