In addition to all that was written above, straight-weight (aka "monograde") oils usually do not (or maybe never do) contain Viscosity Index Improvers (VII's) or Friction Modifiers.
On one hand it means they don't flow as well as their OW-, 5W-, 10W-, 15W- or 20W- brethren at low temps, but on the other they do not shear down to a lower viscosity with use, as do the xW- versions, and they are usually less volatile, if they are made with Group II basestocks, as are Delo 400 straight grade oils.