The American Petrolium Institute (API) standardized the rating system for motor oil and lubricants. It's for the protection of the manufacturer and the consumer as well.
A manufacturer can say there engine requires a CD rated oil. You check your bottle of oil and the API spec is CF. This means not only does it meet a CD rating. It exceeds it.
Take a look at the other end of the scale. 2007 and newer diesels require a CJ-4 rated oil. The CF rated oil you have doesn't meet the needs of one of these diesels.
The same thing applies to the SM, SL rating. The "S" side of the rating applies to gasoline engine requirements. It's very common to see multiple API ratings on diesel oil bottles and not so much when you look at a gasoline oil.
You can run diesel oil in a gas engine but most gas oils can't be run in a diesel. Obviously a diesel engine is alot harder on oil so they require a heavier duty additive package to protect your engine.
Basicly the API made it easy for us to buy a product, find what the manufacturer requires for a lubricant and then look at the API rating on your favorite brand of oil and see if it meets the manufactures requirements without needing to be a chemist. I hope this makes some since and answered your question.
