Yes, it will work on petrol, diesel, but the percentage of improvement differ from one type to the other. The average engine will only require one. However some racing machines have used more than one. Maximum effect could be achieved if each cylinder was fitted with it's own specially on large diesel trucks. No,you should not have to make any adjustment to see the improvement as long as the engine was well tuned in the first place.
Mileage is the sum of the total miles driven divided by the amount of fuel used.
Most people believe they are receiving better mileage than they actually are. We suggest that you drive 1500 miles of normal driving before checking for improvement. Remember it took thousands of miles to build up these deposits in your engine. This condition is only temporally and will stabilize. This depends on the size and type of the engine, mode of driving whether, city or highway, and weather conditions.