The 1.8T in the Mk4 GTI (AWP / AWD and relatives) is a strong engine, but I watch it for two things: coil packs and oil sludge.
Coil packs first — this is the engine that made VW coil failure famous. Misfires, flashing CEL. I diagnose and fix it exactly like any turbo four (see Coil Packs Misfire) and fit the revised coils.
Sludge is the one that actually kills these, though:
- The 1.8T's oil galleries are small and the oil-pump pickup screen clogs with sludge if the oil changes got stretched or the wrong oil went in. Once that screen's blocked it starves the turbo and the bearings, and that's the engine gone.
- I prevent it with oil every ~5k miles on a proper VW 502 00 full-synthetic. On a used one, I drop the sump and look at the pickup screen — a sludged screen tells me everything I need to know.
While I'm in there, the diverter valve, the cam-chain tensioner at the back of the head, and the breather/N249 plumbing are all worth a look.
So buy a 1.8T on its service history — a clean, regularly-oiled one is bulletproof; a neglected one is a time bomb. See GTI Service Intervals.
For the routine side you'll want a VW 502 00 full-synthetic and a fresh filter, and the revised coils if it's misfiring — I've put them below.