I was just reading the manual (thank you Carl) for the new to me (ancient)
HF lathe and it says to fill the head stock with heavy weight motor oil. I
would think something like 80-90 gear oil would be better rather than a
motor oil. What do you think?

P.S. There is about an inch of oil in the bottom of the gear case.
Probably hasn't been full in a very long time.