I did some digging and it seems that about a plastic bottle capful every 500 miles or so. Consider that capful is mixed with what 20 gallons of gas (yes less gas as you drive) that means a drop or two gets mixed with say a gallon of fuel and then sprayed threw the injector. It is probable that over time and many miles that there is some build up. Again how long before you would knew of any difference in performance with the heim, might be hard to tell. Also if it takes 5 years or more, will you still have the truck then? Yes it is say a couple hundred dolllars if you go with a good one, but when you see what people are spending on their trucks why not? Then there is the issue of draining the catch can say every 1000 miles or so. Depends on how maintenance minded you are. But if you check your oil then you could just empty the can at that time. Just my two cents