Always run the highest octane possible unless the manual says not to. The higher the octane the cleaner the fuel burns through the whole system. Parallel it to the chemistry test from high school. The more orange the flame, the dirtier it is, the bluer the flame the more pure it is.
The top tier gasolines are: BP/Amoco, Texaco, Shell, Sunoco, and Chevron.
Stay away from ExxonMobil gasolines. They are detergent gasolines that actually leave a "ring" throughout the lines like a ring around a bath tub after the water drains, thus clogging them up, and any other areas with deposits.
A good raw gasoline to clean your system without a gas additive is Citgo. Alternate Citgo for every 2 refills of the gasolines above and watch your mileage increase after 2 cycles. I've been doing it for over 10 years.