Lubrication for a Clausing VS13 lathe

I have a nice Clausing-colchester vari-speed 13" lathe that has weeped
out a bit of lubricant over the past several years, and has reached the
point of needing to add some more. The manual recommends Shell Tonna 33
and Tonna 37 for the Gearbox and headstock respectively, and yet another
for the apron.
Looks like they're not made anymore. What's the appropriate oil to use
as a replacement? What Quantities am i looking at as well? it's not
listed on the lube chart. Finally, does it make sense to drain and
refill fresh on a regular basis, or just top off?
Thanks,
--Joel