As to your question, the setting is not intended to be used long term, or so I’ve been told. Perhaps a tech will have a more definitive answer.
As a former owner of a ’91 300TE 4-matic, I would strongly suggest selling the car. The 4-matics tend to require endless maintenance for the 4-matic system and for the excessive front end wear that occurs as a result of the 4-matic system. I really loved my ‘TE but after 50+ trips for repairs, I just gave up.