is it legal to use a pre existing fuel cleaning product and improve it and sell it under my own label?"

It is legal to buy a product, repackage it, and sell it. Under patents, the legal right is defined by the Exhaustion doctrine. Under trademarks and copyrights, the legal right is defined by the First Sale doctrine. You can read about either on Wikipedia, which will give you a better sense of whether these doctrines apply to your situation. Without knowing all of the facts of your situation, I cannot tell you if there is something else you are doing that may not be legal. Similarly, I am not sure if 'fuel cleaning products' have any special regulations regarding sales, so the answer is limited to saying the manufacturer of the existing fuel cleaning product would not have any legal action against you once you bought the product from it. Good luck, Todd