Syntetics don't degrade as fast and do a better job than dyno oils at lubricating. This is fact. I was talking with a guy, they have a stock car, by changing to synth gear oil, the rearend temperature was substantially cooler. He told me how much at the time, but I forgot. It was almost 50% cooler than with conventional gear oil.
Now, in an engine, results varies because engines vary. And, sometimes there are reasons not to use it, some will say about lifters requiring friction, the synthetics lubricate too good in some cases.
What is more important than oil is filtration. A good air cleaner and oil cleaner. Especially for guys like me who wheel in the mud and dust. I use a secondary oil filter combined with Esso full synthetic 0w40. If you are using Fram products, you have a problem, Wix is one of the best when it comes to full flow filters, and they make filters for Napa and Carquest.
Another reason for synthetics and long drain intervals is environmental issues. That's alot of oil being wasted when people change at 5000 when you could easily run your oil to 10000-15000. They only people benefiting from this is oil companies and your friendly quicklube spot. An oil analysis will tell you how your engine and oil are doing and give you an idea how you should be dealing with oil types and change interval, given you have the proper filtration system.
Here's the site to learn all about oil and filtration.
http://www.bobistheoilguy.com/
