When an oil is rated 15w-50, the first number is the viscosity that it
acts like when cold, and the second is what it acts like hot.  For many
years, there was but one grade of Mobile One oil -- 5w-50.  When hot, it
acts like a 50 weight oil would when IT was hot.  When cold, it acted
like 5 weight oil -- it did not thicken much at all.  In fact, these oils
don't change their viscosity much at all across quite broad temperature
ranges.

I suspect that many people were hesitant to use 5 weight oil, and the
marketing department madde them produce the 15 and 20w-50 oils.  I have
used Mobile One in my LandCruiser for over 20 years, and more recently
I have used Castrol Syntec.  My Volksies have always leaked too much,
and I'd rather leak cheap oil.  I *think* that some of the newer Vanagons
I own will get synthetic oil next change, though.

As an aside, Mobile One is no longer sold for aviation purposes, and Mobile
had to overhaul hundreds of engines.  Apparently the synthetic oils don't
dissolve lead well, and with the incredible amounts of lead in aviation
gasoline, the lead would build up in some airplane engines, eventually
causing blockages.  It was too bad -- I was about to change to Mobile 
One in my plane when all that broke.  I doubt that lead accumulation is
going to be a problem in VW engines, but the synthetic oils *do* have
some different properties, some subtle ones might be killers like they
were in airplanes.

malcolm