For my '08 6.4 at 156K: Last oil change at 150K, adding Archoil to oil and each fueling.
My results over six thousand miles were not more than .2mpg difference. Read that as negligible. The product may actually be doing some good other than mileage, but how to tell? If you haven't had the engine apart prior to the additive, and then again after some long term use, then anything inside remains hidden.
In the oil: Archoil uses potassium and boron which are apparently harmless to the engine; it just gives off-the-chart readings for their content. A high potassium not associated with Archoil may indicate coolant in the oil.
Also: I am using Rotella T6 for now. I added the Archoil products at the oil change and each fueling. I had it tested at 6000 miles. The Blackstone report indicated my engine is doing very well with no water or fuel in the oil, and doing great wear-wise. I will retest at 8000 when I next change oil and filter.