FWIW I use a 1 liter mixing bottle from RGS.
Similar to the Stens one but with a taller neck so the oil can be measured out more accurately.
So again, just mix what I need even if it means doing several mixes a day.
Any modern ashless 2 stroke oil will be fine.
Not convinced that synthetic is any better than normal oil but considering how little you use the cost is double very little so makes no difference.
In the workshop I usually sell RGS oil unless Stens have a big discount sale when I buy & sell their 1 Liter bottles.