I’m not too sure how effective the one-time stuff you pour down the gas tank would be since it’s rather diluted by the time it reaches the injectors. (If it WAS a really powerful chemical you can be sure they wouldn’t sell it over the counter, just to professional shops.) And gas is expensive enough without spending money on some additive every tank full. Most mechanics use fuel injector cleaner that connects directly to the fuel rail and car then runs entirely off this cleaner for 5-10 minutes for a thorough cleaning, however nothing beats removing the injectors and having them ultrasonically cleaned, flow tested and the spray pattern checked. I’ve used http://cleaninjectors.com on V6 Alfas with good results. It comes out to about $12 to $14 per injector when you factor in shipping.