Can someone tell me why high-octane gasoline is required for a 1.8T engine for the Passat?

Small engine 1.8, so it needs better gas to run--in Europe you pay tax for bigger engines. The engine is tuned up for the gas. Higher octane gas has slower detonation thus smoother and more effective transfer of energy, then lower octane which burns very quickly, making knocking sound on valves.

Should regular gasoline not be used at all?
Also, can the "high" octane gasoline to use, for the 1.8T engine, be the middle grade at the service station, or does it have to be the highest-grade gasoline (high test)?

If you don't use high octane fuel in your turbo-charged car you're killing it faster. Be sure to tell the buyer of it that when you try to sell it someday. Luckily, the 1.8T has knock sensors that retard timing when shitty gas is used saving the engine but drastically effecting performance. Don't be cheap. Use 93 if you can and be good to your car. If you don't want to use the good stuff, then go buy a non-turbo car.

i wasnt aware the 1.8 t required premium....oops...that sucks

I have not been told about the High-octane Gas....Thanks for that...even though I run regular and have had no problems in the 4 yrs I have had mine. I know that you need to run the best grade oil for the turbo, but good note for gas.....Thanks.

I found out about the higher octane, that is required, right after I bought the car, used. The salesman showed me the octane that is needed printed on a sticker inside of the gas cap door. Check yours. Also, it might be listed somewhere in the owner's manual, as well.
You can mix high-test with the middle grade, if you want, to acheive the octane. You can still use a lower octane, such a regular, but I believe the performance goes down some. I don't mind spending a little more for gas to have better performance, especially if that is what the engine was designed for.

Its Better To Use High Octane Gas Coz Youll Have More Power Performance With Your Car Compare To Low Octane Gas That Builds Carbon To Your Valves That Makes It Less Power And Performance Even Though High Octane More Expensive But Its Worth It.

You should run a fuel with the octane value specified by the mfg.
The mftg must assume that you will run the engine at full throttle for extended periods of time. The engine heat build-up could then lead to destructive detonation. To avoid having to honor a warantee claim for stupid driving, the mfg will require the use of a higher octane fuel.

If you only drive in the city, and never fully open the throttle, always drive a low rpm, you might get away with using a lower octane fuel. If the engine is driven at full song you would be best off following the mfg requirements.