Since this isn't flux-core solder, it'll basically last forever. Old flux-core solder often doesn't work as well and needs liberal quantities of flux pen to make nice joints. 20/80 might've found some use for soldering valve/tube mounts, if your facility has been a recording studio for a really long time.
But an 20/80 alloy is so far from the eutectic mix that it's going to have a very long cooling period where it's a pasty slush of molten tin with rapidly growing lead crystals that will just be unpleasant to work with. I think I learned to solder with 20/80 (dad was a radar tech; mention TSR2 and he'll go apoplectic still) and it was dismal. It was a great way of making dry joints. Still, watching those crystals form and having to deal with intermetallic buildup (purple! bright yellow! harder than metal files!) on the iron got me interested in materials engineering.
But really, you should be avoiding lead: lead-free all the way, don't listen to the old guys. Good on you for trying to pass it on in a responsible manner rather than dumping it.
Apart from being a great word to say, eutectics are a neat concept: an alloy of two metals that has a lower melting point than either of them. I forget how this works, but the eutectic diagram (nicked from http://www.farnell.com/datasheets/315929.pdf
) is neat:
It shows pure lead on the left melting at 327 °C and pure tin melting at 232 °C on the right. Only pure lead, pure tin and the eutectic alloy (63/37) go straight from liquid (light grey) to solid (mid grey). Every other alloy goes through a slushy phase (dark grey on the diagram) until it cools below 183 °C, where all lead/tin allows are solid. 20/80 is only fully liquid above 275 °C, and it has to cool by nearly 100 °C before it is solid. That part of the cooling curve is an increasingly sullen slurry until it finally hardens. Blecch.
‘Remember the Golden Rule of Selling: “Do not resort to violence.”’ — McGlashan.