Most of this is easy; bare with me. The numbers denote the S(ociety of)A(utomotive)E(ngineers) viscosity (thickness) classes. Each class is defined as a range and the details can be seen here. This characteristic is NOT weight--all engine oils weigh virtually the same--but most of the industry and even the oilfreaks who populate this board still misuse that term. The higher the number the thicker the oil. The 'W' means Winter or Winterized. It's an old (and probably unnecessary) term denoting oil that will not evaporate or burn off as quickly as the lighter oils that aren't 'W' rated. The 2 numbers used for what's called a multigrade oil are a little more complicated. The first (or only) number is the viscosity range measured at 40dC. The 2nd number denotes the viscosity class the oil has at 100dC, which approximates operating temperature. All oils thin as temperature increases; a multigrade oil thins less than a single-grade oil. Look at the chart again and the 2 left-hand 'Kinetic Viscosities' columns. The 2nd number of a, say, 10W-30 oil means that it has thinned (at 100dC) only to the range associated with the industry-standard 30-vis. oil. Multigrade oils do a better job of lubricating cold engines at startup and hot engines when operating. Hope this helped. One more term. Viscosity index or VI is a measure of an oil's resistance to thinning at high teperatures. The higher the number, the less it thins, so multigrade oils by definition have higher VIs.