MB has used hardened steel inserts in the aluminum heads for a very long time. Other than possibly more often than normal valve adjustments, they should be fine. My owners manual for the 72 280 SE 4.5 says it is fine to use unleaded.
I've never been able to figure out how lead oxide (a mild abrasive) prevented valve seat wear anyway -- I'd have thought that lack of metal oxides ( MMO aside, since it isn't used any more) would make the valve seats last LONGER, not wear them out faster... Hmmm
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!