Want a testimonial?
I've run Mobil 1 in every single one of the cars listed below- each car
had never had synthetic oil as far as I could ascertain.

1981 Jetta
1981 Rabbit
1984 GTI
1986 Vanagon (with 1989 Audi 80 motor)
1987 Scirocco 16V
1988 Ford Ranger V6
1992 Corrado VR6
1994 Jetta GLX
1995 Passat GLX
1996 Jetta Trek

Results?
The Ford seeped a little at the oil pan.
The Corrado seeped a little at the rear main- but there's some debate
as to whether that leak was pre-existing, and whether the actual
problem is that that particular Corrado was a major piece of... junk.
The Vanagon, the 16V Scirocco, and all the cars with VR6 engines would
get 2 seconds' worth of lifter noise if they hadn't been run for more
than a couple of days.
Probably, all of the VW inline engines would have leaked at the front
seals, but I tend to replace those seals as one of the first things I
do when I get a new car, so we'll never know.

Every single one of these cars had some sort of temperature gauge, and
several had one for water AND one for oil.
Every single gauge read lower within 10 miles of the switch to
synthetic, and stayed lower under all driving conditions.
The VR6 cars had digital oil temperature gauges, and they ALL averaged
10 degrees cooler under all driving conditions.
The VR6 cars also had digital MPG gauges, and I noticed (I was looking
for it) that I gained at least 2 MPG in all of them, with a high of 4
in the '94 Jetta!

All that being said... I run Delo 400 in all of my cars now. I like
that my lifters aren't noisy on "dry" starts, and that it's cheaper.

All tests... such as they were... were conducted in Colorado over the
last 6 years, mostly in the Front Range near Denver, and commutes down
to Denver, with some long-distance stuff mixed in.

HTH!

-Simon