You guys have some pretty interesting ideas about oil, that's for sure =).
Synthetics are excellent for turbocharged engines, they resist coking at high temperatures, which means your turbo's oil passage stays clear, instead of becoming blocked by charred oil.
This phenomena (coking) is the number one killer of turbos.
They have extended change intervals because they do not break down nearly as much with contamination from combustion by-products (blow-by, EGR).
They also increase horsepower by reducing frictional losses, depending on the motor, HP gains of 5-10 horsepower are not unheard of when switching to synthetics, but results do vary.
Synthetic oil is not good for all engine types or situations though.
RX-7 and 8 users are not supposed to use synthetics because they do not readily combust in the engine. The Wankle design consumes oil as a normal operation, it lubricates the apex seals.
But the oil must burn, or it will start to gum things up.
Usually you do not want to switch late in a cars life, for the simple reason that your seals will more than likely start leaking badly.
Synths have different seal/swell characteristics than mineral based oils. This also includes your valve stem oils seals, which means you will start burning/blowing more (expensive!) oil.
I'm running AMS 2000 0w-30 synthetic, and the specified change interval is every 6 months or 9000 miles. Oil filter at 6 months, regardless of mileage.
It's about $8 a quart, but worth the money. I drive very hard, and put a lot of miles on my car. Check out AMS's website at:
http://amsoil.com, that can answer more than a few of your general questions.
So do yourself a favor and switch, but switch early!
Justin
'03.5 Mazdaspeed Protege
0w-30 AMS 2000 Oil
Flashed PCM
Medium Tint
P.S.: If you're concerned about the cost of simple oil, you probably don't belong modifying any car, as the old saying goes:
"Speed costs money son... How fast do you want to go?"