[Leica] cleaning rear elements

[Rei Shinozuka <shino@ubspainewebber.com>]

i notice that the rear elements of some of my lenses have a little
schmutz on them, looks like a little grease.  (i bought all of my leitz
optics used).  what's the best way to clean the rear elements
considering that there's an oily brass rangefinder cam ring next to it ?

i've used a microfiber cloth which does a reasonable job on the 35mm which
has a shroud around the rear element, but i'm still
afraid of getting oil from the ring on the cloth.  anyone have a cleverer
method?

thanks,

- -rei