On the sealed transmissions: Yes they are sealed, but they are technically serviceable. My brother 2001 Passat was like this but the factory process was to extract fluid with an evacuator. He sold it recently but in my research, the "stumble" we experienced around idle MAY have been something that could be resolved with changing the transmission fluid. The car had 75,000 miles and at some point a shop fixed a slight trans leak and topped it off...so they said.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000