Closest Circle K to me has Valero pumps, but it used to be a Stop-n-go/Diamond Shamrock before that.
All the new Circle K stores are Circle K branded gas. No info on them at all though. I fill up at them on occasion.
Guess I am lucky though, I have a Valero (it is Top Tier, and where I fill most often due to price), Stripes, Walmart, Kroger, Citgo, Chevron, Mobil (x3) and Shell (x5) within 1.5 miles of me. You can almost triple the number of stations if you go out 2 miles (with 3 Circle K only stations).