I would start with this:

Mercedes-Benz W124 Coolant Flush and Replacement | 1986-1995 E-Class | Pelican Parts DIY Maintenance Article

Using google try:

M104 Coolant flush

and

300SL coolant flush

The Zerex G-05 suggestion is good.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"