An interesting post from the VW newsgroup:

Forwarded message:

]From: finnegan@nrlssc.navy.mil (K. M. Finnegan)
]Newsgroups: rec.autos.vw
]Subject: [W] Antifreeze in Volkswagens...
]Date: 14 Apr 1994 12:49:00 GMT
]Organization: Naval Research Laboratories
]
]I just had a lengthy conversation with a gentleman at Zerex concerning
]the use of their antifreeze in VW's, MBenz, BMW's, etc.  As you all
]probably know, most European manufacturers recommend the use of a
]phosphate-free antifreeze in their cars.  In VW's case, at least, that
]means buying Autobahn coolant from your local dealer.  After my
]water pump replacement this weekend, I put in a gallon of Zerex,
]which states on the front of the bottle that it is "suitable for
]all modern automotive cooling systems."  Dan Simoes and I were talking
]this morning about this issue, so I decided to go to a source.
]Here's what he said in a nutshell (keep in mind that this person 
]works for Zerex.)
]
](1) Initially, the European manufacturer's claims were based on their
]    extensive use of aluminum in their motors (mid 70's).  The claim
]    at that time was that the phosphates in traditional coolants could
]    possibly damage the aluminum.  John pointed out that almost ALL
]    cars these days use aluminum components, and no domestic or
]    Japanese manufacturers seem concerned with the problem.
]
](2) Apparently, the concern now stems from the relative "hardness" of
]    the water in some locations.  When phosphates are mixed with "hard"
]    water, the water is softened (which, incidentally, is the type of
]    water you want in your system), but precipitates form as a result
]    of the removal of the "contaminants" from suspension.  These
]    precipitates are what seem to concern the manufacturers.  John
]    countered with the point that the amount of precipitate is
]    miniscule compared to the amount of other contaminants that
]    make their way into the cooling system, so it should introduce
]    no problems whatsoever.
]
]He finished by saying this:  If your car is still under warranty, by
]all means use the dealer recommended coolant so as not to void your
]warranty.  He said that Autobahn coolant is dark blue, and the
]mechanic could tell immediately (if the coolant were the traditional
]bright yellow/green) that you had not used Autobahn.
]
]If your car is out of warranty anyway, IN HIS OPINION AND
]THE OPINION OF ZEREX you will not harm your cooling system in any way
]by using their product OR the coolant of any other reputable
]manufacturer, phosphate-free or not....
]
]Kenneth.Finnegan@nrlssc.navy.mil

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Jerry Dunham, MSE                                             (512)244-8598
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