I've got to reconfigure some old copper plumbing that was installed in the 70s, so almost certainly were assembled with leaded solder. It will be fairly easy to unsweat the old fittings, wipe the pipes and fittings that are staying in place, and sweat on the new stuff, but I'm wondering if today's unleaded solder will flow and mate with the old solder film left on the pipes. Planning on using Oatey #5 flux. It is almost impossible to cut the pipes back to clean copper.