You only need to put a good mid to high grade petrol in the tank. No worries about lead v unlead as MB has been using hardened valve seats since the 50'-60's. Running no lead fuel won't hurt the valves.
I suggest checking the manual to see what grade is recommended. If no manual, start with the mid and see how it runs. Listen for pinging from the spark plugs. If any ping, bump up a grade.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.