It should mix up on it's lonesome just letting it sit long enough (time depends on temperature). But if your tank is completely full and you put it into the top of the fill pipe, it might take a while to get down to the tank. "For Best Results" being the key term. It just means with a nearly empty tank, you get faster mixing, and presumably a more spread-out concentration. But I can't imagine that half a gallon burned without the stuff makes that big a difference either way. I'd say "marketing" was involved in the language. <img src="/images/graemlins/smile.gif" alt="" />