zone is not important but latitude is.

if you can grow day long onions at your latitude then grow copra for a yellow and red wing. copra stores a year, i've had them 14 months tho that was pushing it while red wing stores probably 10 months.

I grew copra this year and love them. Will definitely grow them again this year. My reds were Red Zepplin, which is a long keeper, but I grew both from plants from Dixondale Farms and will do the same again.

I have just learned(from Farmerdilla), that TEMPERATURE is the most deciding factor as what kind of onion to grow and where.

Onions are cool crops and therefore you cannot expect them to survive 10F and thrive at 105F. Here in this part of GA (zone 8), we have cold winters and short springs. It can get up to high 90s in june. So the onions should be done by then.
Actually they will die.

So then, we have to plant Short-to-medium day variety that by the time the days get very HOT and will stay that way for a while, onions should be done. To do this, we will do winter sowing seeds and sets.

definitely, you will have different conditions in zones 4, 5, and 6. And I guess you will grow LOND DAY varietie.