Doug,
I usually do light cleaning with window cleaner that has a little vinegar in it.

Found this link a few weeks back:
http://www.flickr.com/photos/26262745@N08/3252244170/

Looks like alka seltzer really works wonders. Not sure what the Pentacon/Contax metal is - looks like nickel-plated steel(?), but I suspect the alka seltzer would work as well with aluminium.