Here is a very good website about the chemistry of
octane
http://chemistry.about.com/library/weekly/aa070401a.htm
I e-mail someone at the site to ask where the octane
products where made.   His response was

 "It is a mess.  My understanding is that the octane
> rating is established during the manufacturing
stage.  So each grade is created as a unique
grade.
Hope that helps a little . . . Daniel Emmanuel"

One thing for sure Daniel is right about is that "it
is a mess"

Straight-run gasoline has an octane number of about
70. In other words, straight-run gasoline has the same
knocking properties as a mixture of 70% isooctane and
30% heptane. Cracking, isomerization, and other
processes can be used to increase the octane rating of
gasoline to about 90. Anti-knock agents may be added
to further increase the octane rating. Tetraethyl
lead, Pb(C2H5)4, was one such agent, which was added
to gas at the rate of up to 2.4 grams per gallon of
gasoline. The switch to unleaded gasoline has required
the addition of more expensive compounds, such as
aromatics and highly branched alkanes, to maintain
high octane numbers.

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