When I converted my 81 300D I replaced everything. Worked well. When I converted my 82 300SD I evacuated the system and checked for leaks. There were none. So I charge system per the instruction, 2 cans of R134 then a charge of oil, and another can of R134 held top up till system would not take any more. This AC unit works better than any system I have ever had in any car. I was surprised. R12 is better but my luck with R134 has been positive. Just my .02.
82 300SD 225k
81 300D dead at 207k