I have talked to a bunch of rebuilders that have built engines, run them for
long periods of time with synthetic oil, and then torn them down again.  The
concensus is that synthetic oil definately reduces engine wear.  Also I have
heard that if you change just your filter every 6k miles and just add the
quart of oil that comes out with the filter, that you can extend your oil
change durations from 6k miles to 10k miles or more (but I would ask your mech
his opinion about this first) because the synthetic oil just doesn't break
down like normal oil.
I have heard these great things about Mobil One.  I haven't researched any of
the others so I have no idea if there is a quality difference or not.
Ken Wilford
Van-Again
John 3:16