API Motor Oil Guide

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Aug 10, 2004
High in the Mountains of Central California
Here's the latest API Motor Oil Guide (in pdf format) for those who'd like to read about such stuff from the "horse's mouth."

API Motor Oil Guide, includes SM and CI-4+

Hope it's useful!
"SI" was skipped as well, not sure why. They have this note on the bottom of page 2:

Note: API intentionally omitted “SI” and “SK” from the sequence of categories. For more information about API’s Engine Oil Program, visit our website at www.api.org/eolcs.

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.
What happens to the lettering system when SZ gets old & needs to be revised 30-40 years (hypothetical) from now?

They may regret skipping alphabet letters then.
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