I would use conventional for an engine that old.

    The weight depends on condition of the engine.
You want it heavy enough to maintain proper oil
pressure, and that's it.  Any more is just a waste of
energy.
     I would start with 10W-40, and keep a close eye
on the pressure gauge, both hot and cold.  If pressure
is low, add heavier oil, if it's low, add heavier oil.