synthetic oils leave a protective coating even when the majority drips back to the pan after you kill the engine, whereas dino oil doesnt (at least, thats part of the ad campaign). thats why most of the wear is normally when you start the engine like v8toilet said (with a dino oil, anyway). synth lets you extend your intervals even if you dont drive much, like a lot of high-perf sports cars that spend most of their time garaged, since the engine is still well-protected.
...however, if you were to listen to "popular opinion", using synthetic oils in an engine will cause that engine to sludge, sieze, melt, explode, leak, run backwards and upside down, make your stereo sound funny, and diminish your sex drive. so, of course, will leaving any kind of oil in your engine past 3000 miles regardless of the condition of the oil. old oil in your engine may, in fact, make your toenails fall out. i believe this very strongly, and change my toenails every 3000 miles.
bottom line is your engine will be better protected in heat and cold, wear less on startup, and less likely to sludge if you use a synthetic oil.
-sean
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