I saw something years ago when they were still calling it "Clean Power". The original links for these presentations are dead, but this was the only archived version that I could find. I think there was an even earlier version of this presentation.
https://web.archive.org/web/2020082...ent/uploads/2013/12/Costco-T-C2-CUPA-2013.pdf
This is what it says about the delivery:
• “Batch Controller” in data hut controls the metering and dispensing system.
• Attendant keys total fuel gallons immediately prior to each delivery.
• Additive automatically injected at ~3.25 gallons per delivery.
• Additive is mixed with the fuel by the delivery agitation.
• System has run successfully since April 2011.
I didn't see anything specifies that it's injected throughout the entire delivery to assure uniformity. There's also something a bit off about the ~3.25 gallons per delivery given that all loads are likely going to be different amounts. They also say that the valves used are similar to those used at fuel terminal additive racks.
I'm thinking that you're assuming that the additive is being carefully distributed throughout the delivery so that there isn't just a blob of this stuff dumped into the tanks that needs time to mix. I guess like a soft drink tap where I can literally see the syrup and carbonated water blending. I'm still not sure it's going to be properly blended because it still takes time for that to happen. In a tanker. there's the time (plus sloshing from transport) that eventually arrives at an equilibrium where all the additive is evenly distributed.