Mark,
        The only requirement that is important to keep your wasserboxer running
longer between head replacement is that it is PHOSPHATE and SILICATE FREE. In
most autoparts store there should be at least one brand that boasts this
(Prestone has one, its orange in color to help tell it apart from the others). I
am not sure if the Havoline Extended is phosphate & silicate free.
        The word on the street is that yearly coolant flushes with the correct
coolant is your only ammo against frequent head leakage/head replacement. The
"green stuff" that we are used to seeing corrodes the German aluminum heads
(there is a good tutorial on this under the Problems section at the vanagon.com
site). Just last week Ron at The Bus Depot (best prices I have seen on vanagon
parts + good advice) made the statement that he believes to be true...even with
yearly coolant flushes with the right coolant the wasserboxer heads are good for
about 100k miles!
        If you are in the 90k range you might keep a close eye on this. If you
have some luck on your side, maybe you can be the exception to the rule.

Gregg (Conshohocken, Pa. outside Philadelphia)
'85 Westy
'small fleet of non-VW cars and motorcycles