When I change my oil I use synthetic exclusively, and I never have a problem when I go over 3000 (Oil is never overly sludgey, no knocking or hearing lifters.... etc). I won't run it to 5,000, usually 4,000.
But in doing this, you have to make sure once you go Syn you stay syn.....as I have heard you can have problems switching back.
Syn is expensive but I swear by it and cars are more expensive if they break down..... if a car knocked or burnt oil when I bought it, first thing I do is get a Syn oil change and the problem goes away.
PS after watching the Lube Stop post ( here:
http://www.tfproject.org/tfp/showthread.php?t=105580) if you do not change you own oil make sure the people you take it to are legit and truly using Syn. as the cost is almost double normal oil.