Earlier quoted context omitted.
From the perspective of global warming, all that matters is the net contribution of carbon dioxide from fossil fuels to the atmosphere. The reason oil companies object to the Renewable Fuel Standard is that it results in lower net usage of oil. If it resulted in higher net oil usage, they wouldn't oppose it. So that answers your question.
No, that isn't all that matters. I outlined the other factors that matter in my original comment. Wouldn't you agree that fracking also contributes to global warming? Or that fermenting corn does? What matters is the net total. If all that mattered was CO2 from oil, it would be an easy problem: get rid of oil, use ethanol. Thing is CO2 is the same, whether it comes from oil, ethanol, coal burning power plants, or cow…
Especially if we develop (through GMOs or ordinary selective breeding) crops specifically for fuel purposes to make the whole process more efficient.