Originally Posted by 04Freedom
I drive a lot
High mileage driving is quite benign to oil... If you don't use a synth or synth blend you might want to consider it.
O2 sensor is good to check/change, as is the catalyst which can drop mileage if falling apart or clogging up. Exhaust manifold should be very carefully checked for cracks/leaks, the older 4.0's were infamous for them.
What exhaust system are you running? A decent cat-back, possibly with a low restriction cat, can help a bit. I wouldn't go headers unless you plan to replace injectors to deal with them...
I forget the RPM, but the valve rotation on the 4.0 only occurs at engine speeds well above what it tends to cruise at. A full Seafoam treatment and then an oil change is worth considering.
In over 150K of driving on a 4.0, I was never able to truly crack 20 mph. It has a sweet spot that begins to disappear at about 70 mph and mileage starts going well down not much after that. Frankly a '14 with a hemi gets better overall mileage with the same driving tactics. The 4.0 has very nice low end torque and a torque curve slightly flatter than Nebraska, but a high MPG cruiser it has never been accused of being.