short hand for
American Petroleum Institute Service Categories in gasoline engines it is a minimum standard of quality that oils that carry the API starburst and doughnut must adhere to, the minimum evolves over time, when my LX was produced API SH was the standard and is called for in the owners manual, until recently SL was the current standard, it is being replaced by SM, there is some debate by those who follow this closer than I about weather SM was truly an upgrade,
After posting that I have been reading more about SM vs SL , found this Virgin Oil analysis of and debate about the new Mobil 1 0w-40 SM interesting.
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=11;t=000275
Apparently this was SL anything in the stores now will most likely be SM
in other posts there I have read about GM complaining that some anti wear additives can poison the cats, so the API has called for a reduction in certain anti waer additives, other say this will cause the oil companies to use more exotic additives and base stocks witch may be a good thing,
A similar governing body exists in Europe, the Acea
European Automobile Manufacturers Association witch has a very different rating system instead of just a moving minimum they have grades where oils get a rating based on tests, the harder tests can generally only be passed by synthetics.
Mobil 1 0w-40 was interesting as it carried not only an API rating and also ACEA ratings. These are required for warranty purposes on some European vehicles.
More info
https://forum.ih8mud.com/showthread.php?t=46732
Interesting note, about this time Mobil regrouped their oil line including synthetics, the brand new Mobil one Extended Performance line (15,000 mile) is rated SL. no Acea ratings.
I may have to move to the M1 EP line, need to read and watch soem more. just wish I could find the EP 10w-40 for sale. 15w-50 works but is likely thicker than optimum for my climate. may be perfect for back home in FL though.