Hi Miriam,
OK, I'll just toss it, and next time I'm in a fabric store I'll buy some.
And... guess what... I just tested the 401 again after opening it & cleaning/greasing/oiling it, went from a 6 to a 2 on the dial (6 suddenly felt too tight), and the stitch is amazingly nice (with the "bad" thread)!!! I'll test some more and will reset the dial once I'm certain all's good. This will most probably get rid of my "0 is too tight" problem too.
I still need to learn about the needles & thread weight.
Anyway, thanks again, all of you, for helping me out!