LOL, "really old thread....I mean, REALLY OLD, like from the 1970s." To me that's just not new thread any more. We've got some from the 50s that we've acquired in "grab bag" lots on GW that sews just fine.
Most of it is American made Coats & Clarks, or Clarks or Coats.
My wife commented that the way it's stored has a lot to do with how the thread lasts.
We don't use it for anything important, but ..... we do use the C&C Size 50 thread to set the tensions on the old Singers. In the manual I have for the 66-16 it says to use Size 50 thread to set the tension. And it works too.
So even if we don't use it for anything important, keep it around for that.
Joe