The 2.slow has a nasty habit of clogging it's rings with carbon, and nothing you can do really to stop it. VW built the factory pistons cheap, atleast on this Beetle it was number 4 and the rings were baked, and it only has 180,000Km.
You basically need to use Amsoil Powerfoam, through the intake system, blow it in through a vacuum line running into the intake, while you have a full bottle of their Flush and 2 cups of Varsol poured into the crankcase. Switch it over to Redline 5w40 and use the Passat 1.8t filter M1-301, I have yet to find a better filter for these cars.
That's really the only shot you have at stopping that car from roasting oil, besides rebuilding the engine. Well... You can run Rotella T6 5w40 in it, and accept that it will burn a little.
Edit: The rings on this '98 were all installed right side up... The upside down ring thing is just a cover excuse for very poor engineering, who makes a piston without oil drain back holes for the oil ring... VW sure does, well did every other engine, ditched that feature.