A lubrication engineer told me one time that Slick 50 is bad for engines, because whatever TFE doesn't get caught in the filter is going to break down into different compounds under combustion chamber temps ( ie teflon doesn't remain teflon at those temps - it becomes other things ), and will tend to create deposits on the valves. No idea if that's true - waaay over my head - but he was pretty adamant.
For your reading pleasure, here's an interesting article - be warned, though - I believe the source is an Amsoil pusher, so it is biased. Still makes sense to me, though . . . .
http://bestsyntheticoil.com/dealers/amsoil/snakeoil.shtml
COnsumer Reports did a pretty comprehensive test a few years back ( although it didn't include cold starts, and so was missing a potentially important component ), and was unable to measure any difference between major brands of oil, even using high quality measurement tools and after lots of miles. They did , as I recall, find differences between SPE certified oils and uncertified cheap stuff.
Look for the starburst on the container. Change it at intervals appropriate to your use. If I had a vehicle where it was appropriate, I'd run synthetic, but my high-mileage car uses a little oil ( at 270,000 mi with absolutely no engine work, it has a right - 90 Toyota, if your wondering ), and my wife's car would take 2 years to go 15,000 miles, and there's no way I'm leaving any oil in for 2 years. So, it doesn't make sense for me, but I know some pretty knowledgeable guys that run synthetic, esp with a double-filter setup. But in my opinion, the short term power and mileage gains from Slick 50 may come at a high cost .