All the "flex fuel" rating means is that you can (if you wish) run regular
gas with higher amounts of alcohol blended with it.
The fuel system such as lines,seals and other components have an
enhanced version installed so they won't fail due to the higher amounts
of alcohol used.
The majority of DJ owners on this forum use gas with a max of 10% alcohol
which the refiners do as a normal blend.
In other words don't sweat about the whole FLEX FUEL thing.