Yes, pouring in the additive to a OWB will mix because of the circulation pump. I believe the concern in previous posts was how to mix diesel fuel additive in a storage tank that is already full. I would think a hand crank and a few fuel cans or another storage tank would work. Pump a little out, pour in the additive and pump the fuel back in. I certainly wouldn't do half or even a quarter of a 300 or 400 gallon tank, but anything from 15 - 40 gallons I think would help it mix. I used to have a fuel oil storage tank and there was two access tubes on the top, one was a vent and the other had a spot for a hand or electric pump. If this is the situation, couldn't you put the pump in one side and the filler in the other hole and mix it that way? It might be worth a few minutes of time due to the value of that quantity of fuel IMO.