For years (I'm 50) I changed my oil and filter every 3,000 miles. When I bought a new car (2000 Camaro SS) I started using Mobile 1 and changing it at 7,500 miles. That was a hard transition, but now I use another synthetic (Amsoil) and change my oil once a year! I only put about 10,000 miles on it a year and I change the filter after 6 months, topping off the oil. I've had the oil analyzed, and it is fine. There are several studies on synthetics that say they are good over 18,000 miles and maybe 25,000 miles. That has been hard for me to believe, but it seems true.
Until I fixed the oil leaks in my 87 325, I used conventional oil. It has 300,000 miles on it and I'm debating switching to synthetic oil. I would use synthetic oil in any new car after a break in period. In older cars that leak or burn oil, probably not.
In my Camaro, I picked up .1 in the 1/4 mile by switching to synthetic oil, transmission fluid and gear oil. I autocross the 325 and SS. The SS runs cooler on synthetics.
I'm not very "green", but I do care about the environment. I'm more concerned about depedency on foreign oil supplies. Synthetics and their longer change intervals will help. Think about it.