RE: Substitutes for R12
This subject was explored in every possible way a few years ago on this
list.  Being to lazy to bother to look for the thread or the notes I kept at
the time I'll try to summarize from memory.
First, R134 is what is recommended by VW, full service bulletin on it.
Second, when changing to R134 adapters are screwed on to the connections to
the original AC.  Those adapter are needed because of the fittings used with
R134 AND to indicate that the change has been made. Now, for the Freeze 12
(and a bunch of other such).  There were several strong advocates of  Freeze
12 for use in the Vanagon.  And it apparently works quite well.  But chasing
down a bunch of links that people posted provided the reason that it is not
'approved'. It contains flammable gases in minute quantities and has not
been tested to the point that is required for official approval.  And on top
of that it is specifically illegal for use in passenger cars in a number of
States.  BUT is is okay for non highway use like in farm equipment, heavy
construction equipment, etc.  That's why it is readily available especially
around farming communities.
Another related note on this subject, based on conversations with a close
friend that at the time had been running his own auto repair shop for many
years.  If you load your AC system up with the stop leak gunk, and are
honest about it, then most shope won't touch your AC system.  This is
because  that stuff totally disables the refrigerant recovery systems that
legit shops must use to recover the coolant rather than release it to the
atmosphere.  It had cost him big bucks to get his equipment fixed after such
an incident and the second time it happened he quit servicing AC.  Since
then the makers of the AC recovery equipment have added a detector or some
sort to try to avoid this problem but in talking to one service shop just a
couple of years ago they said the same thing.  If you've added a 'stop leak'
to the AC they won't work on it.