I have a F-350 dually pulling a 5th trailer 39.3 feet. I travel and live in this unit. My question is I had my oil changed first time and ford dealer put 10w/30w Motor-craft. The manual calls for 5W/40W in severe duty. I have been checking on line and local stores and the only 5W/40W is Motor-Craft and Shell Rotella T-6 synthetic oil. I know it cost 2 times regular oil. My question would you be able to get more miles between changes, and better fuel miles. I just want to protect my engine even if it cost more, and extra miles between changes would be nice. How about better fuel miles.