As you can see from the chart, oil quality is an important factor in your decision of what oil to use in your vehicle's engine. You would not want to use a lesser-rated oil in your engine as it could cause premature wear or possibly failure due to the reduced protection. The higher the rating of the oil, the better the quality of the additives package, which means better protection for your engine. While using an SH rated oil in your sixties muscle-car would be a benefit for the life of the engine, using SA rated oil in your new Honda S2000's engine would not be advisable. The number designation found on an oil container represents an oil's viscosity rating.
Viscosity is a measure of the oil's thickness.
Using accepted techniques, measurements are taken at specific temperatures to measure the oil's viscosity number. Oil with too low of viscosity can breakdown and lose strength at high temperatures. Oils with too high a viscosity may not pump through the engine quick enough to lubricate engine components properly at low temperatures. Engine oil should flow easily when the engine is cold and remain thick enough to protect the engine when the oil is hot. Single weight oils are oils that do not change in viscosity.