I wanted to report back on my experience with Exxon Synergy Diesel. At the outset, let me say that I know that this is not a scientific analysis and that it is one person's experience.
I have 89K miles on my 3.0 A6 TDI. Almost all of my miles are commuting miles in the NYC metro area. I have hand calculated the mileage of every tank since new and have input it into an excel spread sheet. (yes I have OCD!). After 89k miles my overall MPG is 28.6563. During those 89K miles I used BP and Shell fuel exclusively. So I have a pretty good baseline for comparison.
In recent months I noticed a decline in my MPG and decided to give Exxon Synergy a try. I have now driven 1,136 miles on it and have averaged 29.2783 MPG, i.e. an improvement of .622 mpg, or an improvement of slightly better than 2% when compared to my lifetime MPG. This is about the same as Exxon claims. These last 1,136 miles were typical commuting miles for me. No extended highway trips.
Of course that it is comparing it to my lifetime mileage. However, remember that I told you that I had observed a drop off on MPG and that is why I tried Exxon Synergy. In fact, for the 6,619 miles before the switch my MPG had dropped to 27.272. The drop off started last August and when it happened I tried various fuel additives to no avail. By late September I stopped with the fuel additives and decided that the car was just getting older. So by the time I tried Exxon Synergy I had driven over 5,000 miles since adding any of the additives. Comparing the Exxon Synergy to the entire "dropped period" the mileage was 2.006 MPG better, ie 7.3% better.
Of course part of the entire "drop period" included the warmer months and the impact of the fuel additives. So I also looked at my MPG for the 1,150 miles before the switch. This roughly lined up with the same number of miles that I had driven with Exxon Synergy (1,136) and all of the miles were winter miles (January-March). During this period I averaged just 25.727 MPG on BP and Shell. When comparing that MPG to the 29.2783 MPG I got with Exxon Synergy, there was an improvement of 3.551 MPG, or 13.798%.
Based on my experience it seems that Exxon Synergy does a better job of cleaning the fuel system than the additives I tried (Stanadyne, Power Service, Liqui Molly and XPD) or the fuel offered by Shel and BP. It also seems to improve the MPG by .6 MPG in my car, or about 2% when compared to the lifetime MPG.
I know that this is not a scientific analysis, but I thought I would report my observations.