I was told by one of my "mechanic" mentors who worked on VWs but
was also a licenced Airframe and Powerplant mechanic that, at least
in aircooled engines, premium was best as it burned slower and 
evenly and therefore more efficiently.

There are some things I just don't scrimp on....oil, tires, brakes
and fuel. I'd rather spend a few extra cents getting brand-name
premium fuel than any other fuel...off brand premium or even
brand-name regular.

I know Joel has his stats on price, and even has some high mileage
cars...so here's my stats running my system:

1970 BMW 2002TI - 200K miles 
1980 Porsche 911sc - 150k miles
1974 VW Thing - 200K miles/90k miles on each engine before anal-retentive
rebuild
1986 GTI - 206k miles
1967 VW Camper - 100k miles

All of these vehicles required NO major engine work...and I would attribute
that to good fuel and good oil (Mobil 1 on all but the Thing) and with
Mobil 1 oil changes every 6k miles with filter and the Thing 2K miles
with Castrol 20w50.

Just my own opinion...

Ric