The OEM plugs are NGK BKUR6ET-10. I and others have been using Autolite Double Platinum because it gives the engine better low-end torque. I can run in 5th gear down to 1500 rpm, no problem, but I remember needing around 2000 rpm with the OEM plugs.
http://newbeetle.org/forums/2-0-liter-gas/60668-spark-plug-help.html
http://newbeetle.org/forums/2-0-liter-gas/60668-spark-plug-help.html