Better fuel economy after an oil change??

[Railrust]

I've noticed this "phenomenon" now for quite some time right after an oil change, my truck gets better fuel economy right after an oil change (lasts for maybe a 1,000 miles). I'm talking an uptick of about 2 mpg for around 1,000 miles.

Just this past Monday I changed my oil and filter (0w20 Rottela Gas Truck 8 quarts 5.3 Chevy Silverado) and I've averaged between 27-28 on my daily 80 mile round trip commute. It's mostly highway and easy back road driving, but yesterday I was stuck in some traffic, did some idling and I thought...there's no way I'm going to average over 26 when I pull into that parking lot. And sure enough as I pull in the dash computer says 27 (and I realize the dash isn't accurate but it's what I go by).

I'm figuring new oil seals the rings better? Maybe new oil is just "clean" without the fuel dilution yet and contamination? And I'm not a Rotella Gas Truck advocate, I only use it because of the rebate...I mean it seems like good oil, but I experience this uptick in fuel economy with any oil, on any vehicle I've owned for the most part. Anyone else experience this?

 

[Mad_Hatter]

The virgin oil is thinner than the used oil, you'll see a slight uptick in mpgs (oil pressure changes too)..plus as you destroy the VM's over time, the oil is less able to resist thinning at high temps...but all that benefit can disappear as the oil ages. Then it's wash rinse repeat with the next oil change.

 

[Railrust]

Originally Posted by Mad_Hatter
The virgin oil is thinner than the used oil, you'll see a slight uptick in mpgs (oil pressure changes too)..plus as you destroy the VM's over time, the oil is less able to resist thinning at high temps...but all that benefit can disappear as the oil ages. Then it's wash rinse repeat with the next oil change.


VM's? Is that viscosity markers/makers?

So it's because the oil is initially thinner, then as it gets saturated with soot, etc, it becomes thicker...but then the oil shears and becomes thinner? This must have an effect on the cylinder/ring seal efficiency, no?

 

[demarpaint]

2 mpg is a big increase. I typically air my tires up when doing an oil change if they need air. Are you airing up your tires too?

 

[Mad_Hatter]

Originally Posted by Railrust
Originally Posted by Mad_Hatter
The virgin oil is thinner than the used oil, you'll see a slight uptick in mpgs (oil pressure changes too)..plus as you destroy the VM's over time, the oil is less able to resist thinning at high temps...but all that benefit can disappear as the oil ages. Then it's wash rinse repeat with the next oil change.


VM's? Is that viscosity markers/makers?

So it's because the oil is initially thinner, then as it gets saturated with soot, etc, it becomes thicker...but then the oil shears and becomes thinner? This must have an effect on the cylinder/ring seal efficiency, no?

VM = viscosity modifiers (synonymous with viscosity improver(s)).

You have a loaded question there touching on a number of things... and I'm probably not the best person to answer you in the first place (ok, honestly I'm just too lazy right now ...‚) but if you want to know about what all these things mean/relate to one another, I would suggest you first make yourself a sandwich and pull up a chair to your Google machine and start googling words and series of words like, "why does motor oil thicken"...or "what is engine soot/sludge" and so on Be prepared, it's a rabbit hole and can consume gobs of your time. Unfortunately, I don't know of one website that has all the info in easy to understand format. A site I do like, if I had to recommend one, would be MachineryLubrication.com

 

[Mad_Hatter]

I guess in simple terms, if I record my mpgs the last 500 miles before changing the oil out...there's a chance that I will see a slight uptick in the mpgs because the virgin oil I just put in will require less energy by the engine to push it around, compared to what it took to push that older, thicker oil around. Now if you use a different oil brand or viscosity between the two intervals, you may or may not experience this a change at all...but all things being equal if you're using the exact same oil brand, viscosity and formula, the chances of you seeing this effect is greater.

 

[Linctex]

I've seen this debated before 100's of times, and generally the person who claims "better fuel economy" after an oil change loses the battle.

The Mobil 1 5W-20 "Annual Protection" in my sump right now has over 16,000 miles on it, and I've been getting better gas mileage these last few weeks (over 19 mpg on E10 in a F150) than I have in the last 6 months!

Lots of people claim stuff happens in their personal vehicles that can't be duplicated successfully in a controlled lab environment.

 

[joekingcorvette]

I have owned many vehicles and never really noticed any difference. Non alcohol gas will give you better mileage but that's a whole new thread.

 

[Linctex]

Originally Posted by Mad_Hatter
I would suggest you first make yourself a sandwich and pull up a chair to your Google machine and start googling words and series of words like, "why does motor oil thicken"...or "what is engine soot/sludge" and so on Be prepared, it's a rabbit hole and can consume gobs of your time.


Oil thickens for 3 reasons:

1) it gets contaminated with particulates (usually combustion by-products)
2) lighter ends are lost to vapor out the PCV valve (NOACK number)
3) It changes molecularly due to extreme heat oxidation.

1) This is rare in modern passenger cars and light trucks that burn so darn clean. Most modern vehicles don't "contaminate" (to the point of detriment) the engine oil until well past the 20,000 mile mark. Many can go much further.
2) This is why we have NOACK testing (and numbers). Unless you track race or tow heavy loads often, your oil usually won't get hot enough to lose any significant amount to vapor.
3) Unless you track race or tow heavy loads often, your oil usually won't get hot enough to have any significant amount of molecular change.

 

[Mad_Hatter]

Originally Posted by Linctex

2) This is why we have NOACK testing (and numbers). Unless you track race or tow heavy loads often, your oil usually won't get hot enough to lose any significant amount to vapor.
3) Unless you track race or tow heavy loads often, your oil usually won't get hot enough to have any significant amount of molecular change.

Because shearing... it's not a friend to oil molecules....‚

Ok seriously, you don't need to tow something or drive on a race track for your engine to exert enough mechanical stresses to tear apart molecules of the base oil and VM (think bearings, journals, cam/crank shaft). As oil ages viscosity changes and if pushed beyond it's ability to resist oxidative and thermal breakdown, sludge varnish and deposits can begin to form - all of these can rob your engine of power and increase wear on parts.

Regular oil changes improve your car's gas mileage as fresh oil better moves through the engine, clears out sludge on components and in passages, better lubricates metal parts reducing heat and friction and helps the engine generally run more efficiently with less work, so it doesn't eat up as much gas.

Is this something the avg driver can notice, maybe... maybe not. In reality, there are so many variables that can negate the benefits of a fresh lube. So in some sense you're probably right, the avg car/driver doesn't notice it. But, in a lab like you said..put the car on a dyno, you'll see the difference. I was pointing out that on paper, it happens... whether or not the OP is actually experiencing this - is debatable.

 

[JoelB]

Generally there are too many factors at play to properly determine why fuel economy changes. Usually it's something more simple like a different gas station with a different fuel source, or weather.

 

[kschachn]

Originally Posted by Mad_Hatter
Ok seriously, you don't need to tow something or drive on a race track for your engine to exert enough mechanical stresses to tear apart molecules of the base oil and VM (think bearings, journals, cam/crank shaft). As oil ages viscosity changes and if pushed beyond it's ability to resist oxidative and thermal breakdown, sludge varnish and deposits can begin to form - all of these can rob your engine of power and increase wear on parts.

I can't imagine any circumstance where an ICE can produce enough mechanical shear to break down a ~C30 hydrocarbon.

 

[kschachn]

Originally Posted by JoelB
Generally there are too many factors at play to properly determine why fuel economy changes. Usually it's something more simple like a different gas station with a different fuel source, or weather.

Correct. The energy density of gasoline even at the same gas station varies, one article I once linked showed up to 4%. Many factors influence fuel economy in the real world and it is not possible to ascribe an observed change to any one variable.

 

[Railrust]

Originally Posted by JoelB
Generally there are too many factors at play to properly determine why fuel economy changes. Usually it's something more simple like a different gas station with a different fuel source, or weather.


Generally I agree, especially with weather, but this just seems a little too coincidental (the oil change thing for me). And it's already stating to change...first day 28.3, second 27.8, third 27.3...I drive a lot, already have 300 plus miles on the oil change.

 

[kschachn]

It's an overused statement, but correlation does not imply causation. Measuring something is only half the battle and being able to conclusively ascribe it to an isolated variable is quite another. You are utterly incapable of doing so in this instance, and that goes for me as well.

 

[Mad_Hatter]

Originally Posted by kschachn
Originally Posted by Mad_Hatter
Ok seriously, you don't need to tow something or drive on a race track for your engine to exert enough mechanical stresses to tear apart molecules of the base oil and VM (think bearings, journals, cam/crank shaft). As oil ages viscosity changes and if pushed beyond it's ability to resist oxidative and thermal breakdown, sludge varnish and deposits can begin to form - all of these can rob your engine of power and increase wear on parts.

I can't imagine any circumstance where an ICE can produce enough mechanical shear to break down a ~C30 hydrocarbon.

Not every molecule in a lube is a long chain hydrocarbon.

 

[kschachn]

Originally Posted by Mad_Hatter
Not every molecule in a lube is a long chain hydrocarbon.

I don't understand, C30 are the short chain ones. The VII are far longer and could be cleaved.

In a former life I was involved with the dispersion of carbon black into a solvent/polymer mixture. We monitored the polymer chain length during processing in the ball mill to prevent excessive shortening the chains and degrading the polymer properties. Even under the massively high impact and shear forces in a ball mill there was never any observed degradation of the solvent which had a similar molecular weight as motor oil. I just don't see how that could ever happen and it doesn't appear to do so, given UOA on straight-weight oils.

 

[DGXR]

If you're seeing a 2mpg improvement with fresh oil, and you always use the same viscosity, that is a serious increase in friction and you are going too long between oil changes. This assumes everything -- including ambient temperatures -- remains the same, which is also unlikely.

 

[Railrust]

Originally Posted by DGXR
If you're seeing a 2mpg improvement with fresh oil, and you always use the same viscosity, that is a serious increase in friction and you are going too long between oil changes. This assumes everything -- including ambient temperatures -- remains the same, which is also unlikely.


I only go 5,000 miles between oil changes using Pennzoil Platinum or Rotella Gas Truck, but you're right, 2 mpg is a lot. Truck only has 35,000 miles. My other direct injected car acted the same way...although it seemed to stop after it started getting higher up there in mileage.

i don't expect the gas mileage increase to last long, but even today in spirited driving it did better than I'd usually get. Temps have been in the 60's this week where I am...mornings have been in the 50's. Historically I see a decrease in gas mileage when the temperatures get bellow 40...this week the weather has been good.

 

[Mad_Hatter]

Originally Posted by kschachn
I don't understand, C30 are the short chain ones. The VII are far longer and could be cleaved.

In a former life I was involved with the dispersion of carbon black into a solvent/polymer mixture. We monitored the polymer chain length during processing in the ball mill to prevent excessive shortening the chains and degrading the polymer properties. Even under the massively high impact and shear forces in a ball mill there was never any observed degradation of the solvent which had a similar molecular weight as motor oil. I just don't see how that could ever happen and it doesn't appear to do so, given UOA on straight-weight oils.

I went back and re-read what I wrote and I see where I incorrectly implied that the short chains break and long one's don't, when in reality (as you correctly pointed out) it's the other way around with the long chains that shear. I know this but the heck if I know why I stated it like that, brain [censored] I guess?...‚).. I also incorrectly implied that shearing occurs to the base oil and that's not accurate. Base oils are rather shear stable (high native VI) and with respect to a multi vis lube, the permanent shearing occurs in the VM itself and does impact the finished lubes 100c kv.

Been a long couple days... and apologize for the confusion. Thank you (I'm being serious) for pointing that out.ðŸ‘.. this is an example of why I always invite critique of what's been said. Thank God this is not my day job or I'd be poor ...‚.