Or a degreaser or solvent.
I'm pretty sure it's lube and not brake fluid (if it *is* brake fluid you'll have the paint peel off everwhere).
My Audi dealer must've gotten sloppy with the lube or something as I'm having this problem with the front wheels of the A8, which just had brakework done (same thing happened when I had the rears done but not as badly). It cleans off OK with just strong shampoo in my case, but that's only because I have FK1000P on the wheels.