I'm quite sure it depends on the specific product. It probably has a dosing based on a certain number of gallons (small 15-25gal or larger 25-35gal). So, mix it in with a full tank, burn off as much of that tank as you can before refilling (so you get the full concentration) and then do it again at the next recommended interval. The weak concentration left over may do some good, but they are usually formulated to dispense their cleaning or lubricating powers in one tank.