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.

Normal driving and a few stops will mix the additive with the fuel in short order.

It's fine and it will mix, just maybe not right away like right that minute, Nothing to worry about.

It will eventually all go thru the injectors one way or another...

At what level does the fuel filler neck enter the fuel tank?

As others have said, the additive will mix with any amount of gas in the tank regardless of when you put in the additive. However, the only concern that I can think of is that...

...some gas tanks have a spring loaded check valve at the tank where the filler neck goes in. And the additive may just sit in the filler neck. It takes the force of the gasoline being pumped in, in order to push open that check valve. IDK if these check valves are still being used today.

I had a Honda Accord with an OE gas tank check valve. I never knew this until years later when I had to replace the gas tank. However the replacement tank did not have the valve.

Just something to consider.

Appreciate the replies. I have a 08 Jeep which has a spring loaded piece at the top but I would not call it a check valve. In any case it has fallen into the tank long ago. Our CTS has no fuel cap, just the push open plastic valve at the top. I don't think either of these acts to trap an additive. This is the first I have heard of a gas tank check valve as you describe. Was this more related to Japanese vehicles or did I just not notice this valve in other cars?

Well, unless you've dropped the gas tanks for repair/replace in other vehicles, you couldn't/wouldn't possibly know.

Those who have issues with the gas pump constantly clicking off while filling the vehicles with gas, may in fact have that check valve that I speak of. I could never fill this particular Accord without issues. It was bad in the cold winter as it took longer to fill. I just couldn't hold the handle/trigger at full blast when filling like with other vehicles that I've owned

2 yrs ago, I dropped the gas tank in my daughters 2006 Mazda3(which she recently sold) for repair. It DID NOT have the check valve and did not have any issues filling the tank. But, I've had issues over the years with other vehicles but, never had to drop the tank so, who'd know?

Most likely you don't have this check valve in your vehicle however, I think it's best to put in the fuel additive/injector/intake valve cleaner FIRST for the reason that I mentioned.