I would recommend use of ballasts rated for T8 or T12 but not both and
to use lamps (bulbs) of the size for the ballasts. Of these two sizes, I
prefer T8, which has 4-footers nominally 32 watts. But I would prefer
F40T12 on a F40T12 ballast over almost anything with a ballast that is
supposed to be good for both T8 and T12, especially if supposedly good for
more than one specific quantity of lamps/"bulbs" or more than one length
of lamps/"bulbs".


And for another perspective...

Fluorescent lights are usually, by definition, mercury-discharge
lights, (even the compact fluorescents.) The small amount of mercury
in the tube needs to be heated to a vapor in order for the lamp to
work properly. They usually perform poorly in cold weather and badly
in really cold weather. Some fixtures get so cold that they won't
even start. A lot depends on whether the fixture is designed to
retain heat.

I don't believe you said what the application was for, but you might
want to consider switching to incandescent or halogen lamps, if the
power consumption is not an issue (i.e. the lights are left on all
night).

For my outdoor (motion sensitive) security lights, I like the old
fashioned high wattage incandescent floodlights and spotlights.

Beachcomber