Costco in Canada sells top tier fuel, true.
Costco does not have an oil refinery, as such their fuel supply comes from various top tier suppliers.
It could be Shell, Coop, Esso, etc.
In severe cold temperatures the octane differences have very little effect on performance.
In my CX 30 turbo I found 93 octane ran smoother with a slightly better fuel economy (negated by the higher price), and if you don’t drive often or have the need to fuel up often it stays fresher. (Apparently).
Every jerrycan I own has premium regardless if it is mixed or pure, the longer shelf life theory is my reasoning.
If I am just going back and forth to work 5 days a week with an occasional trip elsewhere, I can make a tank of fuel last a month.
I run premium, but really don’t notice the performance increases at all.
No dyno, just seat of the pants.