Synthetic oil change interval on 2.8 30V

I have been using Vavoline SynPower Synthetic 5W-30 oil on my 99 Audi A4 2.8. I religiously change the oil every 3,000-4,000 miles but this synthetic is expensive. Everyone seems to be saying 4,000-6,000 miles is safe with synthetic. So I have two questions:
 
- What kind of synthetic oil do you guys use?
 
- How often do you change it?
 
Thanks! 

recently changed over to Amsoil....Lots of info available on this in the forums...do a  quick search..lot of ppl use Royal purple, Amsoil and Mobil 1

Hey;
   The Cummins rep. told the trucking company I used to work for that if they changed the oil any sooner than 15,000 km.(9300 mi.) they were wasting their money. These engines would normally be warm for long hours. They last for more than 1,000,000 km. on regular mineral oil. Those are diesel engines and their lives seem to be a lot longer (double) than a gas engine. The shorter the drives you do the sooner the oil change intervals. According to SAEngineers 80% of an engines wear is in the first 10 sec. of cold start. Audi recommends a change interval of 7500 mi. (12,000 km.)for the 4 or 6 cyl. engine so you would shorten change intervals based on your length of driving intervals. Hope that helps, B. 

I run Mobil 1 synthetic, and i change mine every 6k or so. Oil engineering has come a long way, and standards are changing. Back in the day, 3k was the recommendation, however, with today's oils, 7500 seems to be the norm.

Thanks for the input guys! From what you said and other research I have done online 5,000-6,000 miles sounds safe.

5-6k is definitely safe. especially with the NA engine. Amsoil has a blend that has the longest change interval of any oil. you can run that past 10k