I remember reading somewhere on here that a mass-market conventional 5W30 starts out as a straight 20 WT base oil, then gets doped up with Viscosity Improvers (VIs) and pour point depressants. This treatment allows oil that wasn't 5W30 by nature to behave as if it is in fact 5W30. Neat. Now, what goes into a normal, mass-market, non-synthetic, non-synthetic-blend 15W40? How about a 10W30? A 20W50?