fuel filter is under the car on the driver side should be against the frame, remove the 2 motor mounts on the top and roll the engine forward, if you just want to clean the tank remove it and clean it, i really suggest flushing the system might be really bad inside, as for the oil have no idea jst might the type of oil you are using, is supposed to shift down while going up hill, have your tranny checked, change out your driver side controls and see if that helps if not check the motor on the passenger side, gas milage i only get 17 to 19 mpg if yours is worse than that check your O2 sensors and EGR valve see if that helps, the governor on this car is built in the computer and as far as i know there is no way to bypass this system unless you change out the computer...