On and off yes. To be honest with my varied driving to do with work and moving house a few times, i havnt been able to accurately monitor the mileage since starting. But i havnt honestly noticed any improvement in mpg. The car definitely is smoother and quieter on it, even a tad bit more responsive. I just like knowing that the fuel pump, injectors and combustion chamber etc are being better lubricated and cleaned, especially as im doing part time courier in the car the past while when the car is running all day and getting a hard time. I think it is worth the small extra cost, better than buying the dearer diesel or using things like Redex etc, even though i think redex is ok. If your buying the 5l Comma oil, it works out about 95p per tank at current price, which is nothing really. No negatives to date.