Way back in the dark ages, AMSOIL came out with their first engine oil--10W-40.
For brevity in use, it was tagged AMO (as in AMSOIL Motor Oil, or Al's Motor Oil).
When new grades were introduced, someone--probably Al--figured out which 3 letters to use.
With over 20 different engine oils now, don't even try to make any correlation between most of them. On the other hand, HDD is Heavy Duty Diesel, AME was originally AMSOIL Marine Engine,
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