i am sure i can find more specific, scientific information if i search harder.



but since you asked:

http://www.pelicanparts.com/bmw/techarticles/Borrowed/mult_gas_faq1.htm



here is a small quote from the lengthy article:

4.12 Are brands different?
Yes. The above specifications are intended to ensure minimal quality
standards are maintained, however as well as the fuel hydrocarbons, the
manufacturers add their own special ingredients to provide additional
benefits. A quality gasoline additive package would include:
* octane-enhancing additives (improve octane ratings)
* anti-oxidants (inhibit gum formation, improve stability)
* metal deactivators (inhibit gum formation, improve stability)
* deposit modifiers (reduce deposits, spark-plug fouling and preignition)
* surfactants (prevent icing, improve vaporization, inhibit deposits,
reduce NOx emissions)
* freezing point depressants (prevent icing)
* corrosion inhibitors (prevent gasoline corroding storage tanks)
* dyes (product colour for safety or regulatory purposes)

During the 1980s significant problems with deposits accumulating on intake
valve surfaces occurred as new fuel injection systems were introduced.
These intake valve deposits (IVD) were different than the injector deposits,
in part because the valve can reach 300C. Engine design changes that prevent
deposits usually consist of ensuring the valve is flushed with liquid
gasoline, and provision of adequate valve rotation. Gasoline factors that
cause deposits are the presence of alcohols or olefins ⎺]. Gasoline
manufacturers now routinely use additives that prevent IVD and also maintain
the cleanliness of injectors. These usually include a surfactant and light
oil to maintain the wetting of important surfaces. Intake valve deposits have
also been shown to have significant adverse effects on emissions ⎻], and
deposit control additives will be required to both reduce emissions and
provide clean engine operation ⎼]. A slighty more detailed description
of additives is provided in Section 9.1.

Texaco demonstrated that a well-formulated package could improve fuel
economy, reduce NOx emissions, and restore engine performance because, as
well as the traditional liquid-phase deposit removal, some additives can
work in the vapour phase to remove existing engine deposits without adversely
affecting performance ( as happens when water is poured into a running engine
to remove carbon deposits :-) )⎽]. Chevron have also published data on the
effectiveness of their additives ⎾], and successfully litigated to get
Texaco to modify some of their claims ⎿]. Most suppliers of quality
gasolines will formulate similar additives into their products, and cheaper
product lines are less likely to have such additives added. As different
brands of gasoline use different additives and oxygenates, it is probable
that important fuel parameters, such as octane distribution, are slightly
different, even though the pump octane ratings are the same.

So, if you know your car is well-tuned, and in good condition, but the
driveability is pathetic on the correct octane, try another brand. Remember
that the composition will change with the season, so if you lose
driveability, try yet another brand. As various Clean Air Act changes are
introduced over the next few years, gasoline will continue to change.






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