Almost all additives such as Techron, PRI-G etc. say to add the additive and then add fuel to proper proportion. [1oz per each 16 gallons for instance.]. I get that since the idea is to be sure the additive is thoroughly mixed with fuel being pumped into to tank. HOWEVER, I constantly find myself wanting to add the additive after car is fueled up or shortly thereafter for various reasons. It might be because my wife got fuel and I was not with her or I want to wait until the fuel is used to the point where the proportion of additive to fuel is correct etc. Anyway, is there a problem adding an additive to a partially filled tank in ones garage for instance? I would think that a few quick braking actions shortly after adding would mix the additive well enough. Would appreciate any opinions.