Could just be varnish, largely inert. Most conservative bet is to replace the old lines (but only if you’re in deep on a serious rebuild). Delco eh? Bold move.

That said, if your lines blew clear and you’re pulling a long vacuum, not sure I’d lose much sleep over it.

Thanks Shern. Yeah, It's probably moisture and old mineral oil if I were to guess. My new refrigerant and oil are compatible with mineral, so the oil may possibly re-suspend. We shall see.

I don't plan to replace the lines, no. They are all in good shape other than this stuff on the inside.

Yeah AC Delco compressor! As far as I can tell, they are the best bet for reliability if sticking with R4. I have an older AC Delco compressor that has been great on my sedan for a long time.