I think you are thinking of reformulated gaoline.
http://www.epa.gov/mtbe/gas.htm
http://www.epa.gov/otaq/regs/fuels/420b03003.pdf
from:
http://www.epa.gov/otaq/f99040.pdf
"...
Performance and Fuel Economy
EPA conducted a fleet testing program in 1998 to evaluate car and truck
performance with Phase II RFG, compared to Phase I RFG. Testing took
place in Boston, Chicago, and Houston. The test fleet drove over one
million miles with Phase II RFG. Performance testing was also conducted
in 1998 with utility, lawn, and garden equipment, and with
motorcycles and marine engines. In addition, EPA sponsored fuel
economy testing with Phase II RFG, compared to Phase I RFG.
All available data indicate that no difference in car or truck performance
or fuel economy is expected when Phase II RFG replaces Phase I RFG.
In addition, no difference in performance is expected with utility, lawn,
and garden equipment, or with marine engines or motorcycles.
3
Note that changing from conventional gasoline to RFG, which is oxygenated,
results in a one to three percent fuel economy loss; that is less
than one mile per gallon for a vehicle that gets 25 miles per gallon.
However, there is no additional oxygenate in Phase II RFG compared to
Phase I, so there is no additional fuel economy loss......"
But this would be very much dependant on where you are located. Your summer gasoline may also fall into the category of reformulated, so the winter version may not have more oxygenate.
Some oxygenate might also be added by some marketers for managing water/ice from condensation, like gas line anti-freeze. Although there is unlikely a need for that if there is already some there depending on what type is used.
However they do add more butane in winter which increases the front end volitility to make the fuel vapourize and allow better cold starting. But that isn't anything new, thats been going on since gasoline was a little boy.
http://auto.howstuffworks.com/fuel-e...mmer-fuel1.htm
Depending on how you read this article and it appears to depend on what you consider the norm? Is summer gas just low RVP(vapour pressure) winter gas or winter gas high RVP summer gas. Either way gasoline is adjusted seasonally to make it work appropriately relative the season's operational(and other) requirements relative to engines.
The only time you usually have problems related to seasonal adustment of RVP for gasoline is if you have a really hot day in the spring before the conversion takes place, potential for vapour lock. Or a really cold day before the fall conversion starts which might impact startup or cold idle quality. Being able to see it as a distinct decrease in fuel economy would probably require a substantial mismatch.
But that is normally all managed as well as regional historical temperatures records can predict!
I was curious about the crappy designation? Whether the seasonal adjustments make the gasoline crappy would appear to be more a judgement than a reflection of whether it serves the purpose for the design specifications(whatever those might be, driven for whatever reasons) based on seasonal requirements.
From my own experience I find that the fuel economy of this car varies by really quite a bit depending on how you drive it. The more city driving the worse it is. The more heavy throttle the worse it is. A few spirited accelerations can noticibly affect the average. The longer you stay in lower gears the worse it is. I have varied from 28 to 41 mpg between two tanks(one day apart) just based on the type and place of driving, so it doesn't take much to create a big difference.
BTW we shouldn't forget that its also not unusual to have worse fuel economy in winter just because its colder and takes longer for everything to warm up.