It seems we have a bit of confusion here about octane ratings. People are
confusing the "Research Octane" (RON) rating with the "Antiknock Index"
(AKI) rating which is also know as the "Pump Octane" rating.

My owner's manual (for my '87) indeed specifies 91 RON octane. But it then
goes on to explain that 91 RON is equivalent to 87 AKI, and that it is the
AKI number that we see diplayed on the pumps.

Therefore, Vanagons are "designed" to run on Unleaded Regular.

I have not experienced any detonation problems running mine thusly.
Ironically, my '92 4-cyl Toyota truck which was also "designed" to run on
Unleaded Regular knocked like crazy on Unleaded Regular (when tuned to
factory spec settings), so I had to burn Premium in it. Rather than specify
a more expensive gas in the owner's manual, which might hurt sales, Toyota
let everyone who bought the truck put in Regular, experience detonation,
take it to the dealership for the problem and be told by the service dept.,
"Oh yeah, you have to run Premium in it".

Chalk 1 up for VW.

Also, when I had an aircooled bus, my mechanic insisted that I run Premium
for one specific reason. He felt that when under stress - especially when
climbing hills - aircooled engines are likely to overheat (perhaps only
briefly) to a level that would induce detonation in an otherwise
well-tuned, detonation-free engine. Over a period of time, he felt that
these brief episodes of detonation could be quite destructive to the
engine. By running Premium, one minimized the likelyhood of this sort of
damage occuring.

Paul