I have used MMO, Lucas UCL, and most recently Penzoil Synthetic Blend TCW-3.
I cant tell a difference in mileage with or without them.
However, I believe that the TCW-3 has more detergents than most people think. I have spilled some on myself and it started to burn after a few seconds, chemical burn. I know my skin and carbon deposits are 2 different things, but still...
Also, shopping the Closeout section of Walmart during winter you can find some quality synthetic and synthetic blend TCW-3. I picked up the Gallon of Penzoil I have for $9 back in December or Jan.
Also, in Feb I had my Texas State inspection with the Emissions test done with a tank full of gas and 5 oz of TCW-3, passed it with flying colors.