Give it time. The six qt box will become the standard soon enough. Both sopus and Warren distribution tout their new five qt containers as using less plastic than their predecessors; the next step is for them to follow Chevron's idea. What makes the havoline attractive, at the end of the day, is not that it's six quarts or that it uses less plastic, it's that their selling six qts of oil for the price of their competitors five.
As far as your oil choice, I think using a dexos oil for a pre dexos 3.6 is very smart. Things are rough on oil.