Re: engine break-in
Don't break your motor in on synthetic! It'll take it forever to break in. You want the bearings and rings to seat quickly so that your engine will be up to full compression and stop burning oil more quickly (especially if you rebuilt it with the hard chrome rings).
When I rebuilt my motor, I used the cheapest, thinnest 10W-30 I could find, and I changed it frequently. When you first start up the motor, let it idle for a couple minutes. Then start bringing the idle speed up and down....not racing the motor, but allowing the rings to begin to seat at the entire rpm range.
Then take it out for a gentle drive, constantly varying the engine speed. The best thing to do is put it in 3rd gear, and accelerate from 1000rpm to increasingly higher engine speeds, then coasting back down to 1000 while in gear. Do this from 1000-3000-1000, then 1000-3500-1000, then 1000-4000.....do that as many times as you can, then after the first 50 miles, change the oil.
Gradually bring your rpms up higher and higher as you do the acceleration/deceleration runs, eventually going all the way up to redline. Be sure to keep an eye on your engine temp, and check your oil/coolant clarity for any gasket leaks.
In the first 500 miles, don't do any steady-speed driving...constantly vary the speed of your engine. Row through the gears in town, using the entire range of rpm's you intend to use once the motor is fully broken in. Change the oil again at 500 miles, 2000 miles, and then the usual 3000-mile intervals. You probably won't switch to synthetic until at least 10,000 miles. I tried that, but I burnt through the oil so quickly (in part because of my hard chrome rings taking forever to seat, and my high-pressure GTi oil pump) that I switched back to conventional oil, and at 15,000 miles I'm still using it. Once it stops going through oil, I'll switch back to synthetic.
Fuel economy, I can't speak for. I drive that motor really hard though, because it's like a race car. It sounds cool and boy does that GTi motor pull hard! I can do over 85mph too, but that burns a LOT of gas. I get about 17mpg in my big Samurai with its GTi motor and big ol' Swampers. But my daily driver samurai has returned 32mpg in mixed city/highway driving since I don't have to get into the throttle very much to push through the wind and get the tires rotating.
-- Geoff Beasley