thanks, but that's not what I'm asking. I've done a search and that kind of info is all I'm able to find. All the article tells me is the service type for the particular rating. What I want to know is the actual written specification for "S?" ratings. What makes an oil "SL" as opposed to "SH" or "SD"? Let's say I become an oil blender, just for arguments sake, what do I need to know to make an "SL" or whatever rated oil? What I am interested in is how the API rating has evolved over the last 20 years, or how they have "improved" oil. I'd also like to know what the difference between basestock oil groups. What makes a Group I a group I, as opposed to group II, II+, 3,4,5?