I don't think it will affect your engine longevity. As long as you are nowhere close to the redline, most engines actually run better at higher RPMS. I don't know how high your engine should rev. My 1986 420SEL revs around 3500 at 75mph in third gear, a little under 2500 in fourth. I wouldn't be surprised if this is normal given your engine size. I am assuming yours is a european model. Unless any one can tell you differently I wouldn't worry. One thing to consider is that a high reving, hard working engine like your will benefit from frequent oil changes, even more than a lazy engine, like my V8 or your C230. I would just change the oil more frequently and not worry about longevity.
1995 E420 - retired
1986 420SEL - retired