E85 is a mix of the two. Up to 85% Ethanol and 15% Gasoline.
FlexFuel or E85 vehicles are designed to run on any mixture of gasoline and ethanol with up to 85% ethanol by volume.
So mixing half a tank of straight gas with half a tank of E85 is just an Ethanol mix with a lower percentage of Ethanol. And that's perfectly fine.
You are correct. I must have that part of the owner's manual. Below is the part I remembered and was regarding possible driveability issues.
For best results, a refueling pattern that alternates between
E-85 and unleaded gasoline should be avoided.
When you do switch fuels, it is recommended that:
² you do not switch when the fuel gauge indicates less
than 1⁄4 full
² you do not add less than 5 gallons when refueling
² you operate the vehicle immediately after refueling for
a period of at least 5 minutes
Observing these precautions will avoid possible hard
starting and/or significant deterioration in drivability
during warm up.
NOTE: When the ambient temperature is above 90° F
(32° C), you may experience hard starting and rough idle
following start up even if the above recommendations are
followed.