Adaminak

Changed from Rotella T6 5W-40 to Delo 15W-40 and noticed my mileage dropped about .5mpg. I know oil viscosity makes a difference in smaller gas engines, but with all the rotating mass in a diesel I didn't expect it. Anybody notice a similar change when swapping to or from a lighter oil?

Shorts

Hmmm, it seems like that would make sense. Thicker oil would be harder to push in lower temps. But I see what you mean about the rotating mass wouldn't be as encumbered, but maybe so?

I haven't tried any 5w40 synthetic so I can't compare.

PoorMansCat

could be just a slight increas in frictional losses.... its odd that you could actually see/measure it though... hhmm..

Adaminak

I'm going to run another tank and if I notice the same a difference I'll start to troubleshoot. I changed oil, oil filter and fuel filter. All three are different from what I've run in the past, though I don't know how a fluid filter can make a fuel mileage change...

ripping r

I did the same and with the 15-40 i see about the same. i allways try to run 5-40 cound not get it that time. now i allways keep it on the hand.

bonus

if outside temps have went up a bit, maybe this is your increase?

From 40s and 50s to 70s and 80s, i see 1 mpg increase. The weather just started getting into the 80s, truck mpg likin it TOO

AaronT

Much of the mpg bouncing around is from the quality of fuel we get. It is not all manufactured and distributed equal. An addtion of biodiesel will lower mpg also. Refineries have been adding bio for lubricity. Worse than diesel is quality of gasoline and what state or city you buy in due to the EPA "boutique" fuels policy.

.boB

The differance in oil viscoxity doesn't matter once you get it up to temp (the oil). Whil the oil is still cool/cool, it will make a differance. The engine will have to work harder to move the oil around, and you'll have more frictional losses.

That's the reason many car makers are recommending 0W-20 oil.

If most of your trips are short, it doesn't surprise me that you'll drop a smidgen in mileage. I use 5W-40 year round.

Rednax

Just came across this. Living in South Texas (Corpus Christi) a little farther south than Tampa (and hotter/drier) my switch to T6 from 15W-40 Rotella made for an easier spinning engine at all ambient temperatures during startup.

There's a lot of rotational weight in a CTD.

The same effect was present, though not as noticeable, in 454 Chevys and 440 Chryslers (going from dino to synthetic where oil consumption needed better control AND better hot weather towing duties were needed; the viscosity in both cases was up from 40-range to 50-range; during the 1980's & '90's).

.

Adaminak

Don't know what it was, but two tanks ago I got my best EVER mileage of 21.3mpg. I'm beginning to think I had a bit more crosswind than I was aware of, and it just coincided with the oil change. The next change however, will be going back to Rotella T6 (unless I can get a shipment of Schaeffer's up here first).

DmaxEter

Most all your new class 8 engine manufactures are going to 10w30 and 5w30 for the sole purpose of lower fuel consumption.