I just finished the unit on oil changes with my students. I have told them that with the advent of fuel injection and improved engine tolerances from the factory, 5,000 maximum rather than 3,000 as in the bad old days. Carbs used to dump fuel in the oil and create a lot more carbon. Also pour point depressants have improved significantly eliminating wax build up almost completely. Detergants and dispersants have increased in quality and quantity resulting in cleaner engines. Oil is better than it ever has been. With highway miles, you could probably go to 7,500, but I wouldn't.
The additives in synths are the same. However, synths don't have to deal with parrafin chrysals in the oil, so they don't have to worry about the pour point depressants wearing out. Synths get dirty at the same rate. Realistically, you still should change synths at the same rate as dino because of the contamination and the additives.
If you change the dino at the required rate you should never have a problem with it.
So why use synth? It makes me feel better to spend the money on my baby. In reality, it leaks more and burns more synth than it ever did with the dino. I really oughtta switch back. We'll see how I feel at the next oil change.
If you don't change your oil regularly, you are a fool, or you have way more money than me. The last guy that hit 1,000,000 miles did it in a Chevy pick up and credits changing the oil every weekend.
All the proceding is my own opinion. You should not feel the need to dispute it as you probably will not change anybodies mind. If, you have real data, feel free to post, or send me a link and I will include that info in next years oil change lesson.
if anyone could hazard a guess at what my answer to the poll might be 
Apologies if your particular interval is not listed. 
