You can start here for information:
http://neptune.spacebears.com/cars/stories/oil-life.html and keep looking and you'll find lots of stuff.
I just had my first oil change today in my 2004 Prius at just under 3000 miles. I ultimately chose to go synthetic. I had my oil changed by the dealership service dept. and it cost $18 for that + ~$20 for my 4 quarts of Mobile 1--ultimately about $8-10 more than it would have cost to use mineral oil. I bought my own oil at Wal-mart for $4.77/quart. They quoted me something like $58 for the Castrol Synthetic oil change at Toyota!
It boiled down to this, for me. I'm convinced using synthetic is a little better for the environment than using mineral oil. I'm convinced that it provides a little better wear protection, although probably not enough to prolong the life of the vehicle for the time period I would keep it--IOW, I get a new car every 6-7 years anyway and I have no doubt mineral oil is more than adequate for any car up to 200-250K miles--maybe synthetic will take you to 300-350K on the engine, but I'll be darned if I'll ever own a car that long.
The minor, annecdotal, improvement in MPG is not enough to make up the difference in cost, but I felt that the cost was marginal. The annecdotal improvement in HP isn't enough to justify it either. But, in aggregate, I felt that the environmental impact, MPG impact, HP impact, and wear & tear impact were enough to justify a high tech product in a high tech vehicle like the prius. At this point I haven't decided if I'll push my luck with 7500 mile oil changes which would help balance the cost of the oil by extending the time b/w changes...but I probably won't b/c I'm afraid I might have warranty issues if I haven't gotten them done 'by-the-book' every 5K miles. Once I'm out of warranty I'll probably extend to 7500 though.