Blitz,  as already has been pointed out R-12 is a superior refrigerant to
R-134.  I also subscribe to the conspiracy theory behind the banning of R-12.
 Much research seems to indicated that this is all based on bad and profit
biased science, but what are you going to do.... This is America and you have
to take the few bad things with the many good things.

In any case,  I performed an R-12 to R-134 conversion on my 1990 Oldsmobile
last year in conjunction with a complete AC system replacement (except the
compressor which was a GM "R-134 ready" compressor).  My experience was as I
expected-  it did not cool as quickly or efficiently and the compressor
worked harder.  Overall it worked fine for my A/C needs in Maine.  If I were
back in Texas it would likely have been unacceptable.

There are 2 significant reasons for NOT doing this conversion in your
Vanagon.  First, as I mentioned, your compressor will have to work harder.
It is already old -this could push it over the edge then you will be spending
money.  Secondly,  -and more importantly in the short term-  the R-134
molecule is much smaller than that of R-12.  If there are leaks or even weak
spots in your old system they will be big leaks with R-134.

My van has required a recharge nearly every year as long as I've owned it.  A
recharge costs about $125-$160 depending on region and refrigerant
availability/prices.  In my mind its a cost of being cool... R-12 will be
available for a long time -I budget for the annual expense and think bad
things about the individuals responsible for the current arrangement -who
ever they are!

Good-luck... I hope this is helpful
Eric Small
So. Portland, Maine
'86GL Weekender