I use "synthetic" oil in my vehicles, but frankly I don't really know why I do in the Lexus vehicles. I do not believe that synthetic oils offer significant real-world longevity benefits vs. a top quality conventional oil for 95% of users. Particularly so long as the oil is changed often enough -- say, 6K miles or maybe more, depending upon the driving habits. This is contrary to what most people beleive after years of marketing and advertising by Mobil 1, Royal Purple, and the like, but from what I see from experts who don't have a financial interest in promoting synthetic oils, I believe it to be true.

I can't count the number of 200K+ engines I've encountered that were run on conventional oil. For example, my family has two trucks -- 5.7 liter GMC and 5.9 liter Dodge -- with 270K and 240K, respectively, and both engines still run perfectly well, neither has ever had a lubricated part replaced, and both have been on the "lowly" Pennzoil conventional oil their entire lives. How long will they continue to run? Don't know. Would they run longer on synthetic? Perhaps. I'm sure one could post on a Mustang or Camaro site and find hundreds of similar examples from guys who have such vehicles in their families, and probably post on the Toyota board and find some with 2X that kind of miles. But bottom line is, modern engines will run a LONG, LONG time on a good conventional oil.

I also find it interesting that Blackstone Labs, http://www.blackstone-labs.com/ people who make their living analyzing oil, say:
What's the best oil to use?
Ah, the million dollar question. We are an independent lab, so we don't make recommendations. It has been our experience that oil is oil, and either petroleum or synthetic-based oil will work well for just about any engine.

Come on, you're holding out on me. I should use synthetic, right?
Buddy, you should use whatever you want. Synthetic oil won't guarantee a longer engine life any more than my eating organic food will guarantee I'll live until I'm 90. We here at Blackstone generally use regular petroleum-based oil because honestly, it works just as well for us.

Also a LOT of interesting reading in their newsletters:
http://www.blackstone-labs.com/newsletters.php