MPG difference: 5w30 and 5w40?

[lars11]

Will there be any difference in mpg using 30 or 40 wight oil in my jeep 4.0? If so, will that difference be bigger if I go to 5w30 synthetic?
Will I notice? I drive 5w40 Castrol GTX now and thinking of switching to something else. The jeep manual says 10w30 but I want a 5 cold weight in sweden.

 

[Bobert]

I really depends. Most of the time going thicker will decrease you mpg slightly (1 mpg maybe less). Some though have seen an increase in mpg, but definitely more rare. The thicker oil might contribute to better ring seal if your motor has lots miles on it and might benefit from a thicker grade.

 

[TR3-2001SE]

I have tested mobil 1 5w-30 vs 0w-40 and can find no difference in milage. The forty weight is a lower viscosity 40 and is know to thin out slightly. I have also tested mobil 1 5w-30 against a mixture of mobil 1 viscosities that comes out as 5w-40. This seems to have almost no effect also. I am guessing you would need to go from a 0w-20 to a 15w-50 to notice any difference at all. Oil just has very little affect on milage. Tires and their pressure are the biggest milage variable.

 

[wavinwayne]

In my VW GTI 1.8T, I've used 0w30 syn., 5w30 dino, 5w30 syn., 10w30 syn., and 0w40 syn. I didn't notice a MPG difference between any of them.

 

[lars11]

Ok. One thing: Why are manufacturers moving to 20 and 30 weights? I notice that a good hdeo 5w40 is much cheaper than any 0w20 or 5w30?
I just bought a 0w30 PAO only synthetic from the Swedish Agrol brand (farmer's stuff in big packs). Hopefully it will make cold starts less damaging if theres no mpg gain to be found. Maybe 12000 km OCI too...

 

[Anduril]

They do it to meet some arbitrary EPA standards so they don't get hit with a fine.

 

[mdocod]

the jeep 4.0 isn't an engine I would worry about fuel economy on. It's always going to be somewhere between ouch and bad. 2 valves per cylinder and that huge long crank. Just doesn't do fuel economy. From what I have heard, people with V8 swaps on their jeeps get the same fuel economy or better than the inline 6 did. I would choose an oil that keeps it quite and smooth and/or returns a good UOA.

From what I have heard, jeep 4.0s are great candidates for HDEO oils like the 5w40... I'm betting GC 0w30 (being a thick 30 weight) would be great in that engine as well, especially in winter months.

 

[BearZDefect]

It must depend on the engine and vehicle. I don't have a Jeep but from posts I've read here it may not make a noticeable difference.

In my old Honda Accord, on the other hand, it surely would. I once use a 12.2 cst 10w-30 and consistently lost 2mpg on the highway. After 2,000 miles of this I dumped it (left the oil filter) to go back to a 10.x cst 5w-30 and got back my regular 29mpg. It was warm weather, and the AC doesn't even work so those weren't factors. Same old tires, etc.

My Accord has an oil-coolant heat exchanger which may reduce the possibility of a more viscous oil running at a higher temperature to compensate... who knows.

 

[CR94]

Originally Posted by Anduril
They do it to meet some arbitrary EPA standards so they don't get hit with a fine.
In Sweden?

 

[SwampSurvivor]

While my Jeep 4.0 was still a roadgoing vehicle, I ran everything from 5w-20, to 15w40 in it. The only difference I noticed was more noise on the 15w-40. There was zero difference in fuel economy.

Woah ... just realized this was a very old thread

 

[JLTD]

Originally Posted by Miller88

Woah ... just realized this was a very old thread

yeah 2007! That might be a record for revival.

 

[dubber09]

I ran 0w-40 German Castrol in both cars last summer as opposed to staple PP 5w-30 and had very slight gas mileage improvement ... go figure...

 

[kschachn]

Originally Posted by dubber09
I ran 0w-40 German Castrol in both cars last summer as opposed to staple PP 5w-30 and had very slight gas mileage improvement ... go figure...

You did because of other factors, not because of (or in spite of) the oil. Gasoline itself can vary in energy content by 4% even at the same gas station irrespective of winter and summer blends. Considering that and all the large environmental factors in real-world driving you'll never be able to discern and attribute a change to the oil.

 

[PimTac]

Your right foot will have more impact on fuel economy than any oil.

 

[demarpaint]

Originally Posted by kschachn
Originally Posted by dubber09
I ran 0w-40 German Castrol in both cars last summer as opposed to staple PP 5w-30 and had very slight gas mileage improvement ... go figure...

You did because of other factors, not because of (or in spite of) the oil. Gasoline itself can vary in energy content by 4% even at the same gas station irrespective of winter and summer blends. Considering that and all the large environmental factors in real-world driving you'll never be able to discern and attribute a change to the oil.

Good points. You can see increases in mpg just by airing up tires in some cases.

 

[dave1251]

Originally Posted by PimTac

Your right foot will have more impact on fuel economy than any oil.



Yes it does but there are dunces whom state 40 grade and E-0 provide 30% better gas mileage than up to E-10 and 20 grades and these people are forum members.

 

[ofelas]

15w40 + B0 give me better fuel mileage than 5w30 + B10

 

[dubber09]

I didn't change my routine thus made the comment, you can speculate all you want.

As far as speed goes, I see almost 10% difference in gas mileage driving at 110km/h vs 95km/h.

 

[barryh]

To get back to the original question. There has to be a difference in MPG between oil grades but it seems by most reports to be too small to measure reliably because of all the noise of other variables. In the controlled conditions of lab testing it would be measurable.

I have seen an increase in idle speed after switching to thinner oil on an old engine with no regulation of the idle speed other than the engines internal losses. Even that didn't translate to an easily identified increase in MPG.

The manufacturers clutch at these fractional improvements because they sum up to something when they also put thinner oil in the transmission and final drive, use single row timing chains and smart alternators etc etc.