Thinking of adding a 7 series to the driveway. Already own two 6 series. What kind of gas milage are you all getting with the 3.5, 4.0 and 5.0 liter engines? Also been picking up on some engine problems with the 5.0 liter motor. How is the 4.0 liter working out? I know the 3.5 is basically bullet proof. TIA