Ed, they found out it increased fuel economy by 300% so the oilk companies sent it all to a secret storage facility on the dark side of the moon.
SI is also missing, as well SO be missing in the future, but those are obvious.
A question you might be able to answer... Could it be a tradmark issue?
Gee, I'll have to find that storage facility. Now that there's no more green to collect . . . oh, nevermind, best not to start down that road. :wink:
Seriously, I hadn't thought of trademark; that could be it. I assumed SI was passed to avoid "numerical confusion," and it's a safe bet that SO will be passed in like fashion.
The SAE documents state that SI and SK were used by other marks and therefore skipped over. Now it could be that SI is a trademark of Sports Illustrated and SK is a tool line as well, just to name a few possibilities.
Yes, the SK service grade designation was skipped to avoid any confusion with the Korean base oil supplier, SK Oils.
And I think SI is commonly used for spark ignition and falls into the guidelines to skip any letter that could generate labeling confusion.
777, they'll probably go with something like "SAA, SBA, SCA..." or some doubling up variant.
By then, I'll be in my late seventies or eighties (as many of us will) and we won't care. Let the youngsters figure it out!