My wife's VW Jetta uses syn oil and their recomended interval is 10,000. On the more modern engines with all the electronics you get such a clean fuel burn that you have very little carbon passing into the oil. That is the big killer of oils. Even with extended oil change intervals with modern engines and today's oils you see very few oil related engine failures. Good filters and good oil can run a lot farther than the recommended miles. Lot of that is to get you back in the dealership. I have ran non syn shell oil for 60,000 miles in big trucks, just adding makeup oil and oil samples every 12,000 miles. Never a oil related problem. Engines still running strong at 600,000+ when we sold them.