I can definitely say that my 1985 300D runs much quieter and smoother on the Diesel HPR than it ever did on regular diesel. That is after just the initial 7 gal fill. I added another 6 gal today since I went by the station. It is so quiet now that you can barely tell it is a diesel with the windows down, sitting at a stoplight. Before, at 65 mph I always heard the engine too much. Now, I don't even notice the engine with the radio on moderately. I don't know if it is the higher cetane rating or it helped clean out my injection pump. I know the injectors were already clean since I removed them a year ago, and they were pristine inside. Downside, the price went up in a week from $2.69 to $2.99/gal.