Used Oil = less MPG... Any Proof?!?

Status
Not open for further replies.
Joined
Dec 31, 2016
Messages
10,136
Location
Waco, TX
I have about 9,300 miles on my OCI. (Frankenbrew, prob close to 5W-30-ish)

I filled today & calculated 18.1 MPG, about .2 mpg better than my "running average" of 17.9.

Has anyone actually done any REAL studies that says used oil = less MPG?
The motor oil companies all "say so".... but I don't think I've seen anything legit.
 
I read in Car Craft way back in the day that a fresh synth OC was worth 10 hp. Drag racers in restricted classes would change their oil after a few passes to maintain the effect.

I know that is not really proof, but this was during the early-mid 2000s when synthetic oil just started to become mainstream.
 
The only reason I would believe that claim is when oxidation pushes the viscosity WAY up. That would take, what? 15? 20? 30k miles? Even at that, what are we talking about, 0.3%?
 
Agree. The marketing guys are extrapolating oxidative thickening to a concept more palatable and arousing to the consumer, Horsepower.
 
I have run tests with my Duramax and Toyota Corolla with oils that claim to increase mileage. You could never prove it by me. A five to eight PSI change in tire pressure did thought in fact a couple MPG!
 
Last edited:
I have done 40 plus runs with checking my gas mileage new car old oil.

New oil= 29.4 to 29.8

Old oil= 28.4 to 28.8

And it was remarkably consistent. Every time past 4k miles running Pennzoil Ultra or Castrol gold bottle 5w30. Or around 2700 miles for other group 3 oils. Same road on Rte 5.

Doing something once, twice or three times is not meaningful to me. But having done it 40 plus times... It is and was shockingly consistent fall off. Pretty meaningful to me due to how many times I did it and how consistent it was as well.
 
Last edited:
Originally Posted By: JohnnyJohnson
A five to eight PSI change in tire pressure did thought in fact a couple MPG!


Yes, the difference in heavy cars/light trucks is VERY pronounced.
The change seems smaller with lighter cars.
 
Originally Posted By: bbhero

New oil= 29.4 to 29.8
Old oil= 28.4 to 28.8
And it was remarkably consistent. Every time past 4k miles


How odd.....
I would not have guessed that.
4,000 mile oil is usually still in tip-top shape. (ESPECIALLY Pennz U or Castrol GB)
 
Shear, thermal cracking and fuel dilution would be expected to go the other way.

This implies that, for a lot of oils over quite a high proportion of an OCI, any effects might cancel out, until you got to fairly terminal oxidative thickening.
 
Seriously Linctex.

Strange but a consistent pattern in both my Ford Fusion and my Nissan Altima 3.5. An interesting note is that Pennzoil Ultra and Castrol gold bottle both held up maximum gas mileage longer than any other oil I ran in either car.

All of the other oils started dropping between 2500-2800 miles.
 
Originally Posted By: maxdustington
I read in Car Craft way back in the day that a fresh synth OC was worth 10 hp. Drag racers in restricted classes would change their oil after a few passes to maintain the effect.

I know that is not really proof, but this was during the early-mid 2000s when synthetic oil just started to become mainstream.


I can vouch. For the 1st 500 miles after changing my oil the engine is more responsive and pulls harder. The first couple fill ups after oil changes are pretty poor mpg wise as a result.
 
Originally Posted By: bbhero
I have done 40 plus runs with checking my gas mileage new car old oil.

New oil= 29.4 to 29.8

Old oil= 28.4 to 28.8

And it was remarkably consistent. Every time past 4k miles running Pennzoil Ultra or Castrol gold bottle 5w30. Or around 2700 miles for other group 3 oils. Same road on Rte 5.

Doing something once, twice or three times is not meaningful to me. But having done it 40 plus times... It is and was shockingly consistent fall off. Pretty meaningful to me due to how many times I did it and how consistent it was as well.

Change oil in winter/spring by chance?
 
Last edited:
Ensure that the tests are all done at equal ambient temperatures. The density of air can vary up to 10-13% in summer vs winter in cold weather states.
 
Originally Posted By: Linctex
Originally Posted By: bbhero

New oil= 29.4 to 29.8
Old oil= 28.4 to 28.8
And it was remarkably consistent. Every time past 4k miles


How odd.....
I would not have guessed that.
4,000 mile oil is usually still in tip-top shape. (ESPECIALLY Pennz U or Castrol GB)


I believe Chris142 has indicated that, in order to pass CA smog, they often recommend an oil change. The new oil is better able to absorb blowby it seems, and thus engine out emissions is improved. If so, then I wonder: perhaps once the oil "loads up" with these lighter compounds, and no longer absorbs it, then any and all blowby is simply rerouted back into the engine. But this blowby now acts as "negative BTU's", or causes ignition timing to be pulled slightly, something, which causes a net loss of power (and thus an increase in fuel consumption). The oil could very well be still very good at what it does primarily (lubricate stuff).
 
Originally Posted By: maxdustington
I read in Car Craft way back in the day that a fresh synth OC was worth 10 hp. Drag racers in restricted classes would change their oil after a few passes to maintain the effect.

I know that is not really proof, but this was during the early-mid 2000s when synthetic oil just started to become mainstream.


I recall that article, but you're a bit off base. The HP gain was contrasting 10w-30 syn to a 20w-50 dino. They were used to using the "thicker is better" stuff for high HP motors, and wanted to see what gain could be had for drag racing, where every single HP is a potential win or lose. It had absolutely nothing to do with worrying about fuel economy, wear, or anything else. Thinner lubes cause less parasitic loss; we all know that. But changing oil at every drag race is a fools errand; there's no way oil degrades THAT fast.

To that end, I don't know that "clean" oil will run any more efficient in terms of economy. The only real risk is that if the "dirty" oil was allowed to get super thick due to insolubles/oxidation, that it grossly altered the vis, from running it WAY too long (far longer than any normal BITOGer need ever worry). As long as your lube stays in grade, used oil won't be appreciably different than new in that regard.
 
Last edited:
Originally Posted By: dnewton3
The only real risk is that if the "dirty" oil was allowed to get super thick due to insolubles/oxidation, that it grossly altered the vis, from running it WAY too long (far longer than any normal BITOGer need ever worry). As long as your lube stays in grade, used oil won't be appreciably different than new in that regard.


That is pretty much what I was thinking.
 
Status
Not open for further replies.
Back
Top