Since we are on the subject I thought I'd mention my experience
with the Dow-Corning leather preservative that was put out in
the 1980's.  It was supposed to be the living end in leather
preservative, and in fact it did a good job on the large pouches
under the pipes in the Reproduco that runs on about 5" of wind.
 However, it did slightly stiffen the leather, turned it a rusty
brown color and yet it still took glue for the pipe valves.  I
wouldn't be inclined to use it again, because rubber cement (or
nothing) seems adequate for pipe valves in our clean and moist
Seattle air.  Note, this is probably NOT the same product that
Craig B is using.¶
  As for mink oil, it was advertised as keeping leather supple
and soft, but the organs playing on lighter pressure (like 2.5
inches) pressure soon refused to play.  The pouches were so
stiffened by mink oil that they wouldn't open on low pressure.¶
   --Add'l note, I took apart the 70 year old Duo-Art, and the
leathers were still good.  Luckily, I found no trace of
rubber-dust, so simply re-sealed them with rubber cement.
Worked fine.  I did tear one apart to see how much dust was
under it, and to see if the leather was fragile.  Plenty of
dust, but very strong leather.