I'm also going to do a similar A/C job soon, a new parallel flow condenser, A/C flush, convert back to r-12 from r-134a. The compressor can't be power flushed... if it needs to be cleaned it must be removed, new oil poured in/out a many times, spinning the compressor by hand. Looks like a huge PITA to r/r the compressor and I'm not going to do it, it's a sumpless compressor so there shouldn't be much oil in there anyway. As for picking an oil, I wouldn't use PAG, since it's not compatible with r-12.. If you find r-134a doesn't cool good enough for Boston (it probably will be good though) then you can go back to r12, for $15 I got my EPA license from ase, $60 on eBay bought 2 12oz cans and 1 14oz can of freon. Synthetic compressor oil like BG frigiquiet2 seems to be good as well.

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1980 240d
1999 SL500