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The booster goes into the tank before you add the fuel. Pop off the cap and pour in the amount you need, based on the back of the bottle. This process ensures that the booster will get mixed into the gas properly, as pouring in the gas will stir up the booster. Once you've got the booster in the tank, all you need to do is add the regular gas on top of it. Keep in mind that your octane level will be the octane number of the gas you're buying plus the amount the octane booster ups your level. Therefore, if you're buying 93 gas and you add a booster that increases it by 3, your level will be 96. Higher octane gas burns more slowly, making it less likely to knock, which is only beneficial if your car needs a higher octane level. You'll get the best performance out of the product if you use it within a year. While some boosters will last longer than a year, it goes bad faster when it's not stored in ideal conditions, such as not in its original container. Some boosters are light sensitive, so it's important that you keep them in the original container. Also, follow any other recommendations on the back of the bottle about storing the mixture. When adding an octane booster, it's a good idea to use them up quickly. If you have a car that you don't use much, only add as much as you need for the amount of fuel you're planning to use. Many boosters are ethanol based, and as they sit in your tank, their drawbacks become more of a problem. Buy what you're going to use in the short term, and try to mostly use it up. For instance, ethanol isn't as efficient as other types of gasoline, and that becomes worse over time. It's not as efficient because it burns more slowly, causing a slight reduction in the power.
Pour the booster into the gas tank. Add the gas. Use the booster within a year. Keep the booster in the original container. Try not to leave the octane booster in your tank for extended periods.