[ars]

> Usually additives are added to premium which wouldn't be in mid-grade

That's the perception (and they are happy for you to think that), but I don't believe it's actually true.

And in any case, if it is true, a mix would simply have a lower level of additives for midgrade and that seems right to me.

> although there are older gas stations and cheaper stations which mix high and low on-site at the pump for medium.

I'm almost 100% certain that almost all gas stations have just two storage tanks and the pump mixes them.

See: https://blog.gasbuddy.com/posts/Gasoline-pumping-from-the-st...

http://hyperbear.blogspot.com/2006/05/dirty-little-secret-ab...

[rdl]

Ah, it's apparently 90% have two tanks. The remaining 10% have 3 and have pre-mixed, because they had leftover tank from when they did leaded gas, and by doing this they don't need to upgrade the pumps. So it's probably old/cheap/un-upgraded stations which have 3 tanks.