The oil will never burn away or fall off, it will attract dirt/mud/grime and just become a greasy coating forever. As mentioned above, fresh leaks will be shiny and "wet". If it bothers you, buy some degreaser and go visit the local DIY carwash and go to town on it. You can get it very clean again with some elbow grease and a good degreaser.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)