Swapped to synthetic oil in my 2007 6.0 and lost fuel economy

Last month in Boise I finally switched from conventional to full synthetic on my work truck, and my average dropped from 14.2 to 13.1 mpg over 800 miles. Everyone online swears synthetic is a no-brainer, but has anyone else seen worse numbers after the change or is my injector stiction just messing with me?

2 comments

2 Comments

wyattramirez1d ago

That drop is pretty noticeable, but it might not be the oil itself. My old 6.0 did the same thing when I switched, and it turned out the thicker synthetic was leaking past some tired injector o-rings until they swelled back up. Took about 1,500 miles before the numbers came back to normal, then it actually got better than before. Your mileage might vary, but I'd check for any new wet spots around the valve covers or fuel rail first. Also, keep in mind winter blend fuel in Boise around this time is a known mpg killer, so that could be part of it too. Give it another tank or two before you panic and drain it out.

zaranelson19h ago

Ugh, my buddy had the EXACT same thing happen with his 6.0 last fall! He switched oils and watched his mileage tank for like two weeks straight, then it just came back and even got a little better. He was ready to dump the oil out but his mechanic told him to chill and give it time, and sure enough the truck evened out after about a thousand miles.