SI is an International unit of measure.
SK was skipped because there is a brand of oil - huge refinery - in Korea that is named SK Corporation and they have sold SK oil since they changed their name from YUKONG in 1998 or 1999.
Hello Lone, I do not know why they skipped the SK
rating but I do understand why they skipped
the SI rating, the I in SI looks too much like a 1 (numeral).
I haved worked in the a few fields of facility design and layout, and have worked with building plans,
layout, etc. and the letter 'I' is left out of
column lines used in building plans to eliminate confusion with the number 1, for example, in a row of building column lines on building plan drawings, the columns listed will be A thru H, skip I, then columns J thru Z are listed (if needed).
I have seen the 'I' eliminated in a few fields of engineering and the API being in a lubrication engineering field, my guess is that is why the SI rating is skipped.
I wish it was only trivia to me. They are across the street from me and compete. They were already telling people that they were better than SJ (with their SF/CC oils) since they were SK.
We have another competitor (YPFB) that sells a motor oil labeled "Premium SL". The API did not consider this a problem big enough to avoid the category. It is SF/CD