I read up some more on the whole Duracool stuff. There is some
interesting reading, and what is available from various COMPANIES
varies. However, from the US EPA site this information was gleaned:

It is illegal to replace R12 in a system with a hyrdocarbon refrigerant.
It is illegal to mix refrigerants.
You can replace R12 with R134.
There are no restrictions on replacing R134 with hydrocarbon
refrigerants.
It is illegal to vent (to the atmosphere) any refrigerant....except for
hydrocarbon refrigerants (which due to their flammability would be more
hazardous to reclaim then vent.)

Note that information again. You CAN use Duracool (or other hydrocarbon
refrigerants) if you have FIRST converted to R134. (My instructions as
previously posted did mention that step, and did include the comment
that after the conversion you have your system properly evacuated and
the R134 reclaimed, if not in so many words.)

I've been extremely pleased with Duracool's performance and cooling
capabilities and (obviously) don't hesitate to recommend it to other
Vanagon folk.