Synthetic Oil Question

[Mark Blum <HobbesCT@ix.netcom.com>]

Hi all,

I just posted this in the automotive groups but I figured I would ask it 
here as well.

There has been much hoopla over synthetic oils and their benefits.  I have 
a question though that I have not seen addressed.  All synthetics talk 
about extended oil change intervals and how the oil holds up much longer 
than regular oil.  I understand how the oil doesn't break down as quickly, 
maintaining it's viscosity, etc.  However, oils not only provide 
lubrication but also contain detergents and additives to help protect 
against contaminants and by-products of the combustion process.  One reason 
non-synthetic oil needs to be changed regularly is because these additives 
become depleted as well as the small particles too small to be filtered out 
by the oil filter.  So, my question:

How is it that synthetic oils can be used for longer drain intervals 
without replenishing their additives?  Do synthetics contain more of these 
additives than dino oil?  It seems hard to believe that regular oil gets 
depleted in 3-4000 miles but synthetic can go anywhere from 10-25000 miles 
(according to which company's claims you read).  And what about the 
build-up of the tiny particles?  In my new A4 I plan on using synthetic oil 
exclusively.  I have always been a firm believer in changing the oil every 
3000 miles, but that will get expensive with synthetic oil.  But I am 
having trouble believing I can leave the oil in for extended intervals with 
*no* adverse effects.

Can anybody shed any light on this issue?  I never see it mentioned in any 
of the synthetic literature.

Thanks,

Mark Blum
'97 A4