I know that opinions are like buttholes - everyone has one, but IMO - the primary reason for going to a synthetic versus dino or blends is to extend your oil change interval. Based on experience with UOA, I think 5000 miles is extremely conservative for a full synthetic. Protection properties are virtually unchanged at that interval. As I understand it, many European manufacturers recommend 10K intervals from the factory with full syn.
Personally, I intend to do a 3750 mile service interval with a synthetic blend, although I'm fairly confident that even a decent quality dyno oil with a quality additive package like Castrol GTX can easily do 3750 without significant protection property loss.
FWIW, I think the key to keeping an engine protected, even under severe duty, is not neccessarily which oil you use as much as it is sticking with the same brand and type. The goal being to maintain the benefit of the additive package the manufacturer uses.
Of course, if you drive in consistent stop and go conditions and do mostly short trips, you should definitely go with a full syn and keep your intervals conservative (5K).
If you want to do some interesting reading on the subject, from experts in the field, check out
http://www.bobistheoilguy.com