it's mostly because of the condensor; it's designed for R12 and doesn't work very efficiently with R134. some people have retrofitted a parallel flow condensor that works better with R134. i've heard people having good results with Freeze 12, and from my personal experience i have used Envirosafe which worked better than R134a but still not as good as the R12. i now only use R12 in my W126 and W116 because it works the best

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL