Iceman, I take it that the ratings you are really referring to are the API (American Petroleum Institute) ratings - such as SG, SH, SJ. These refer to performance levels, as oils may have more than one performance level. For automotive gasoline engines, the latest oil service category includes the performance properties of each earlier category. If an automotive manual calls for an API SH or SJ oil, an API SL oil will provide full protection. Diesel engine oils, however, may not include the performance properties of an earlier category.
Here are some of the ratings for gasoline engine oils:
SL - Current - For all automotive engines presently in use. Introduced on July 1, 2001. SL oils are designed to provide better high-temperature deposit control and lower oil consumption.
SJ - Current - For 2001 and older automotive engines.
SH - Obsolete - For 1996 and older engines.
SG - Obsolete - For 1993 and older engines.
SF - Obsolete - For 1988 and older engines.
SE - Obsolete - For 1979 and older engines.
SD - Obsolete - For 1971 and older engines.
SC - Obsolete - For 1967 and older engines.
SB & SA - Obsolete - For older engines. Use only when specifically recommended by the manufacturer.
As technologies advanced over the years, vehicle manufacturers stipulated certain levels of engine oil performance to optimise the vehicle's perfromance. So, even though NP, Shell, Mobil, Castrol or Quaker State are producing SL rated oils, they do meet the requirement levels, however, their performances do vary by brands and depend on the use of the correct viscosity grade.
Which takes me to my next point...
20W-50 isn't a rating. It identifies the oil as a multigrade oil. Multigrade oils are widely used because - under all but extremely hot or cold conditions, they are thin enough to flow at low temperatures (for instance, on start up) and thick enough to perform satisfactorily at high temperatures. Vehicle requirements vary. Monograde oils are just one straight grade, for instance SAE 30, SAE 40, SAE 50. These oils are usually recommended for use in older engines. In modern engines, multigrade oils are more effective for start ups as the engines are more compact and have finer spaces that the oil is required to lubricate. For instance, on cold starts, a 20W-50 will flow faster to lubricate and protect your engine's components than a 'straight body oil' like an SAE 50. In Ferrari engines, the oil is required to flow through passages thinner than your finger nail. Think about it this way: which flows faster - water or honey?