Just for fun I asked a guy I know that used to sell oils and he responed
this :

If you�ve always used mineral oil  with long intervals between oilchanges
and have some consumption - stick to the mineral

The reason is that synthetic oil are more agressive to "dirt" in your engine
so soot and other small particles that might have helped aging gaskets could
be washed away.
Also be prepared to change oil sooner the first time since the engine get a
better cleaning..

The synoil and most of all the Mobil 1 is an exellent oil in the aspect that
it works better the more load you put on it, which is why you so often see
racecars in the commercials, high rpm�s great heat and much load and poor
lubrication due to cornering brings out the best in the oil.
It�s a bit overkill on most streetcars though, you will be just as happy
with a cheaper synthetic or mineral oil.

When I asked him about the Vanagon in particular he recommended a semi
synthetic but were very thorough in pointing out that today all good quality
oils are just good quality and that there is no risk in mixing them too ( if
you accidently fill up synthetic low level with mineral).
The most important thing is to change often, shorter intervals if you only
drive short distances, and to make sure that you get the right viscosity for
the outside temperature in the area where you�re driving the most....

Calle