Delvac or Rotella?

don't think that's it

 

Delvac is one of the better oils. So nope.

 

Prob put bad fuel filters in

Recall!

 

What viscosity was the original oil and what viscosity did you put in @ change? If you started w/ 10w-30 and went to 15w-40, that could make a difference. Also check the oil level on level ground. if they happened to over-fill it, that could also make a difference if the crankshaft throws are beating into the oil because the level is too high. That also aerates the oil and makes it less effective as a lubricant, so increased friction from that makes overfilling a double whammy.

 

It's not going to make a difference... and the viscosity of the oil certainly wouldn't make that big of a change in your fuel economy. Look somewhere else. Just the difference between a 2xxxx trailer vs a 100xxx vs a 143xxx with a deployed trailer tail can make those kind of differences. Run a trailer out of balance with most of the weight on the trailer tandems can do that as well... as can a good dose of cross-wind.

BTW... if you're just reading it off of the ECM display, then you're guessing. There are so many variables when it comes to fuel economy there's no way you can really judge using that as a measuring stick, and it's very subjective as well. Track your mileage tank by tank, using a logbook- writing down the fuel added to the same spot in the filler neck every time, and the mileage run.

Better yet, get yourself an account here...

https://www.mygauges.com/login

...its free, and track your fuel mileage that way.

 

"It's not going to make a difference..."

Well, according to Cummins, it does: The difference between 10W-30 and 15W-40 in an ISX15 is 2-3%. Personal experience with a 2 gallon+ overfilled crankcase on a 60 Series Detroit (courtesy of Speedco) was about 1/2 mpg. It also showed on the oil pressure gauge w/ about a 5 psi loss of oil pressure from the foaming (aeration) of the oil from the crank throws beating it. All that was missing was an egg and some water and we'd have had mayonnaise. Another thing: The better mileage you're getting to begin with (8.5 is great) the greater impact these small additional loads on the engine will make, as they are a greater portion of the demand horsepower required to power the vehicle down the road. Add it up: 2% of 8.5 = 1.7 tenths and 5/10 (this was on A 5.8 mpg truck to begin with, so by comparison it would translate to an additional 2/10s, making it 7/10) for a total of nearly 9/10ths from just those two things: viscosity and overfilling. I think that corresponds w/ what the OP said. Although I don't know if that was his problem, all I was saying was that it COULD have been, and I still think it's possible that's what it was.

 

These trucks are company vehicles, and Prime only authorizes basic service... most likely 10W40. Even if you go from that to full synthetic, you're only going to see around a 2% difference. That isn't 0.5 mpg.

 

"...you're only going to see around a 2% difference...That isn't 0.5 mpg."

No it's not. But it is about 1.7/10, which is what I said I said.

 

Well... it's not going to cause these guys to go from mid 8's to mid 7's... 2% is a loss of 0.2mpg in the mid-8's. That's nowhere near 0.5 to 1.0mpg. It's much more likely considering the variety and hard service the differing trailers they pull, climactic variations, etc. plus the fact that the refineries are pouring all of the left-over no. 1 diesel into the winter-blend mix before summer arrives, that one of these issues is causing the perceived loss of economy.

@michaelg... if the situation persists, report it to road assist and suggest they might want to do an overhead and have the charge air cooler pressure tested. Get that out of the way, and see if things improve.