SN/CF is more or less the generation or oil. We have moved all the way up from SH SL SM etc etc. I think The SAE set requirements that the next generation of oil needs to be and deem if an oil meets that specification. AFAIK the rating takes a lot of characteristics of the oil e.g its ability to resist thickening with age.
I think the current spec is SN.
Heres something I found notice how the SN beats the older SM spec everywhere.
Quote:
The API SN category is an improvement over the API SM category in the following areas:
•High temperature deposit protection for pistons
•Better sludge control
•Better seal compatibility
•Aftertreatment compatibility
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http://www.oilspecifications.org/articles/api-sn.php
Motor oils are a funny thing and if you want to have a better understanding of them go through motor oil 101-109 on this forum. ITs motor oil explained relatively deeply but in layman's terms, enjoy
http://www.bobistheoilguy.com/motor-oil-101/