My dpf levels consistently drop on the highway. It won't drop to 8 percent, but it will drop 8 to 15 points from the starting point. Eventually reaching 80 percent. This usually takes 1500 to 1600 miles. I would also mention urea is not injected into the dpf. It is injected into the scr. These are 2 separate aftertreatment systems. They are usually 2 different canisters, but can be housed in the same canister in some instances su h as the ecodiesel.