Fuel Mileage Down after Changing Oil???

[truckmaniac]

A good friend of mine did his second oil change on his 2014 PowerStroke.

He went through the factory oil (I'm assuming 10w30 Motorcraft), then changed to Rotella T5 10w30 and thought he'd settled-in on Rotella T6 5w40 full synthetic.

He's a stickler for maintenance and monitoring his fuel mileage. Ever since going with the T6 around 4,500 miles ago, his fuel economy has been consistently DOWN .6 to .7 mpg.

He took the truck to the dealer a few days ago and after running various diagnostic in the shop and road testing/monitoring for over 100 miles, they found nothing wrong with this truck other than:

.....Air pressure in tires was set at 70 psi. The door sticker recommends 80 psi.

.....They saw the reminder sticker on the windshield noting that he was running Rotella T6 and said he should go back to a 10w30 Semi-synthetic or Synthetic blend. They went on to tell him that the 6.7 (especially in the warmer climates of Texas) doesn't perform as well on the heavier 40 weight oil.

Will using a 5w40 Full Synthetic actually reduce your fuel mileage, as compared to using a 10w30 Synthetic Blend?

I look forward to hearing your comments and theories.

 

[sledhead_24_7]

It is possible, you say it was his first oil change? Maybe the rings have not seated all the way yet?

I've been running T6 in my '11 6.7 since 25,000 or so, now at 80,600 ish. I have not lost any mileage. But I am also in Wyoming so temps are quite a bit different.