Don't sound good. If you do need a compressor you need a proper flush and re-oiling no matter what refrigerant you use. While I would never convert an early 300e, the compressor should do better than 14months if the thing was oiled properly. The compressors on early (-89) 300Es are too small to do well even without the conversion. I'd put it back R12.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician