In the crankcase, B-12 Motor Tune-Up can actually serve two different functions depending on the circumstances. In engines with excessive deposits, the product will work as a flush by aiding in the cleanup process. In such cases, the fluid may become overwhelmed by these newly-liberated deposits, though, so the fluid and filter should be changed shortly after addition. In engines that are newer, well-maintained, or otherwise expected to be fairly deposit-free inside the crankcase, the product functions as a long-term additive by providing additional sludge and soot dispersancy (especially important in diesel engines!), oxidation resistance, lubrication, and moisture removal. A visual inspection of the oil before and after adding B-12 Motor Tune-Up is often sufficient to determine how the product has functioned—short-term cleanup or long-term additive. That said, it certainly isn’t unusual to be surprised by the amount of color from soot and sludge that an oil has picked up after running the product for just a few minutes. It would be wise for your friend to at least consider doing an oil and filter change if the engine runs so much smoother.