I was interested to know what are the difference between brand names
of gas.  For example, how much difference is Shell's gas from 76's
gas.  I've heard that they use different additives, but is this true. 
If so, what is the effects of the different addi tives and would it be
worth using one brand of gas for the health of one's car?  Is there a
place where you can find out what is in certain brands of gas?

I worked at a gas station the Gasoline itself is the same. They do
change formulas during the summer and winter. a "cleaner" gas for
summer the additives are added at the time the trucks are loaded i
beileve and it's something like 2 or 3 Gallons of additive for a 9000
or so gallon load of gasoline. Yes the additive is different. Places
that don't have their own additive brand just go with the "cheapest"
or easiest to get additives for their gasoline. but like mobil i
worked for (was a privatly owned) their delivery company wallas
transport delivers some of their gas to shells and picks up gas at
shell distrubution points phillips 66 and sometimes others sometimes a
truck is hired for the day. or not regular company sends deliveries.
wallis for example would hire out other company drivers to haul loads
if they  are short on trucks etc. I always wondered looking at
gasoline invocies how much 2 or so gallons of additive per 9200
gallons of gas helps with additive as far as how much additives REALLY
do for your car. I hope this comment helped some..

Years ago, I was told by an Oil Company Executive here in the UK that
there was absolutely no difference between the different products
because the Oil Companies actually pool their supplies.

Of course, the different Companies always try to differentiate their
products through marketing campaigns.

Let me add a comment about 10% ethanol in many gasolines today.  The
advantages are that it will reduce emissions from your vehicle and it
reduces the cost of the product by volume - but it will reduce mi/gal
by about 3%.  Also it is more corrosive for your vehicle than 100%
gasoline.  Just some things to keep in mind when buying gasoline.

From the time it is crude oil in the ground, to when it is sold to the
consumer, gasoline changes ownership on average 7 times.  All of the
gas companies are buying and selling the same thing as they buy and
sell it from each other many times before it gets to you.