This has been a hot topic for many years and many manufacturer franchise owners have warned their customers NOT to use additives or else it will void their warranty. Well not only is this a false statement but it's wrong and you can report them to the Federal Trade Commission, yea...the FTC.
Federal Law - The Magnusson Moss Act

Here's the bottom line: Vehicle manufacturers are not allowed to void the vehicle warranty just because aftermarket parts or additives are on or in a vehicle.If, and that's a BIG 'IF', the engine fails, the manufacturer has to prove, aside from all other causes, that it was the additive that cause the failure.Let's take it an extra step, where do the manufacturers stand? See the pic.