Should I add Floetrol / XIM Latex Extender to my Exterior Acrylic Latex Paint?
Rules that I live by for painting doors:


always paint in the shade, and not when it is too hot (>75°F)
always remove doors and lay flat for painting
always brush doors, no rollers (unless "back-rolling" after brushing, for texture)
do not skimp on paint when applying
always maintain a "wet edge"
always use professional grade brushes- emphasis intentional, this makes all the difference


You purchased top grade paint, there should be no need for additives at all.