Success with 134a depends on the condenser and aux-fan arrangement. More condenser and more airflow is required to get good performance. If you have the original coil and fan setup, stick with R12 or use one of the hotly-debated alternatives.

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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!)