I should also point out that with a converted R134a system, the head pressure on the compressor can be nearly double that of the original R12 setup, so your compressor will be put through twice as much wear and tear as if you were to just leave it as R12. So we are talking about way more than an "oh-so-slight" performance loss.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280