Thx again!
The partial dealer fill-up is now down to just above the "red zone" of the fuel gauge (100km est. range by the trip computer), so it will get its first proper fill-up tonight. Now indicating 6.1 l/100km "since last fill-up" according to the display.
I'm really glad to have avoided premium-fed options. Costco used to have premium 8 cents above regular, now it's 12 cents. Of course, no diesel at Costco, and locally diesel is not advertised as it is on the rural highways, so we'll see what sort of pricing strategy is applied at Shell.