The UOAs posted on here have definitely convinced me that synthetic is not an absolute necessity anymore. I've used synthetic oil almost exclusively ever since 1991, but have recently switched my wife's car over to conventional. Ever since I bought that car in 1999 I ran nothing but synthetic in it. But I truly think I will get just as good (if not better) results running conventional changed every 5-6k as I was getting with synthetic changed every 8-11k.
I still run synthetic in my Corvette, since that's what they recommend, although at the same time the LS1 engine also came in the Camaro & Firebird and they didn't specify synthetic for it. So I probably could safely run conventional, although since I do like to drive it hard once in a while, which heats up the oil, I feel safer with synthetic in there. Plus I also want to follow the oil life monitor in this car for my oil changes, and it's calibrated based on synthetic oil.
I think that ever since the SM/GF-4 oils came out, the gap between synthetic and conventional is considerably less. Ten years ago there was a much bigger gap in quality.