Synthetic Oil Change Intervals?

[Stephen Pitts <excel@aegis.nitco.com>]

I just put Castrol synthetic in my 95.5 S6 @ 10,000 miles on the odo.
Can I go 7,500 miles (Audi maintenance freebes) with the synthetic, or 
do you guys thing I can harm turbo and or rest of the moving parts?
I could be keeping this car till it drops, as I drive 25K miles per year 
on the highway.  What about in January, when my car sits at the airport 
in the sub-zero temps for 2 days?

I would greatly appreciate anyones comments...

Steve Pitts
95.5 S6
94 100S Wagn