Questions about gas octane and fuel in the oil

[MikeySoft]

I know fuel in the oil is not good but I have some additional questions anyway.

1. Someone told me that time lowers octane. Meaning that if the gas is a few months old, its octane is lower. Is that true?

2. If true, would lower octain in the oil lower the oil's viscosity more than highter octane?

3. Would a long run on a highway evaporate any fuel in the oil?

4. If 3 is true, would the viscosity of the oil go back toward normal?

 

[kschachn]

Originally Posted By: MikeySoft
I know fuel in the oil is not good but I have some additional questions anyway.

1. Someone told me that time lowers octane. Meaning that if the gas is a few months old, its octane is lower. Is that true?

2. If true, would lower octain in the oil lower the oil's viscosity more than highter octane?

3. Would a long run on a highway evaporate any fuel in the oil?

4. If 3 is true, would the viscosity of the oil go back toward normal?


The octane rating of straight gasoline is determined by the configuration of the octane molecule, so no it will not reconfigure due to age. However, if there are additives to the gasoline that raise it's apparent octane rating, and those additives can somehow evaporate out of the gasoline or otherwise dissipate then I suppose it is possible. But the alkane will not change on a molecular level.

The same answer applies to fuel dilution, it won't matter if it is gasoline and gasoline only. However, RFG can contain various amounts of additives such as ethanol, so that would affect the viscosity differently than for pure gasoline.

As for the last two I don't know for sure but people on here (and Blackstone) have said that a long run that gets the oil up to operating temperature will drive off fuel and water. But I think it takes longer than most people assume.

 

[Colt45ws]

Oil temperature will drive off some of the fuel, but fuel is made up of many components and some of those have higher flashpoints than what oil will typically run, so it won't totally evaporate everything off.

 

[danthaman1980]

Originally Posted By: MikeySoft

1. Someone told me that time lowers octane. Meaning that if the gas is a few months old, its octane is lower. Is that true?

Perhaps...
"Octane Rating" is a ratio that compares the burn rate of a batch of gasoline to a standard molecure, which is known as iso-octane (2,2,4-trimethylpentane, to be exact). It might be that the octane rating is impacted by time. Gasoline is made up of a mixture of hundreds of different types of hydrocarbon molecules, which are blended together to give the desired octane rating. Time would have a different impact on different types of molecules in the mixture; lighter hydrocarbons might evaporate first, which could actually increase the octane rating. At the same time, aromatic molecules and unsaturated hydrocarbons may start to oxidize, which would decrease the octane rating. Cumulatively, the effects would somewhat offset each other but the net impact might be that the octane rating decreases.

Originally Posted By: MikeySoft

2. If true, would lower octane in the oil lower the oil's viscosity more than highter octane??

Not necessarily; it kind of depends what type of molecules were added to the gasoline to improve its octane rating.
Long, straight chain hydrocarbons would have a higher viscosity but lower octane rating.
Short, highly branched hydrocarbons would have a lower viscosity but higher octane rating.
Short, unbranched hydrocarbons would have lower viscosity and lower octane rating.
Aromatic molecules might have a higher viscosity and higher octane rating.
From a practical perspective, though, it would be difficult to measure the difference.

Originally Posted By: MikeySoft

3. Would a long run on a highway evaporate any fuel in the oil?

Generally, yes.

Originally Posted By: MikeySoft

4. If 3 is true, would the viscosity of the oil go back toward normal?

Yes.

 

[Ramblejam]

Originally Posted By: danthaman1980
Cumulatively, the effects would somewhat offset each other but the net impact might be that the octane rating decreases.

Don't get excited though by seeing the higher octane number; volatile light components are what goes first, and while the portion left behind has higher density/higher octane, it's unavailable for cold starts/high revving, etc.

 

[BeerCan]

I think gasoline because of its nature does not have a single boiling point so the answer to #3 is "some"
Plus I think that mixing with the oil might also change some of its properties. As well as the fact that if the rate of dilution is higher than the rate of evaporation you will always be behind the curve.

I say this because I routinely did 300 mile runs with my 2011 EB and it always had a high % of fuel in the oil.

 

[Ramblejam]

Originally Posted By: BeerCan
I say this because I routinely did 300 mile runs with my 2011 EB and it always had a high % of fuel in the oil.


Here's the important question: What octane have you been using?

 

[kschachn]

Originally Posted By: BeerCan
As well as the fact that if the rate of dilution is higher than the rate of evaporation you will always be behind the curve.


Yes, it would depend on why and how the fuel is getting there in the first place. If you are seeing fuel dilution due to cold weather ops with short drive times then a long drive will help to remove the fuel. But if it is due to a mechanical issue such as a stuck fuel injector then you'll never get the fuel out by driving.

 

[danthaman1980]

Originally Posted By: Ramblejam
Originally Posted By: danthaman1980
Cumulatively, the effects would somewhat offset each other but the net impact might be that the octane rating decreases.

Don't get excited though by seeing the higher octane number; volatile light components are what goes first, and while the portion left behind has higher density/higher octane, it's unavailable for cold starts/high revving, etc.


Well that is very interesting! So in that case, octane rating tends to increase for at least the first for 5 weeks. I would be interested to see what happens over 3-4 months or so. I would wager that octane rating would eventually peak and begin to decrease.

 

[MikeySoft]

Originally Posted By: Ramblejam
Here's the important question: What octane have you been using?

I believe the manual states premium gas is REQUIRED, not just recommended as some newer cars do.

I always put Shell premium when I drive it which is almost never. She always put premium from the closest station which is usually a Petroil station. Petroil is a lower price gas in the Boston area.

We both stopped filling it to full, usually to about 3/4 tank when the tank is about 1/4. This is because she drives so little we don't want the gas to get old or out of season (summer/winter).

I asked these questions because of an UOA report with high copper.
Link to UOA.

 

[Danh]

The fuel octane issue seems to be more relevant in preventing fuel dilution of oil (at least in DI engines like your Accord). As some posters have demonstrated, premium fuel seems to result in less "over-fueling" when a high-load, low rpm situation is encountered. Given the fuel economy mania among automakers, low rpm, high load situations are not unusual. Over-fueling in the engine's way of avoiding pre-ignition.

And while a long highway trip will evaporate fuel, as I understand it oil that has been exposed to fuel dilution never gets completely back to normal because gasoline damages the viscosity index improver additives in all oils, making them less effective.

I have the same engine in my CRV and am fighting a fuel dilution issue as well. My experience seems to be that high speed highway driving using 87 octane gas contributes to rather than reduces fuel dilution. My plan is to start using 93 octane fuel on the same route and performing UOAs to see if it makes a difference.

 

[Blueskies123]

Originally Posted By: Ramblejam
Originally Posted By: danthaman1980
Cumulatively, the effects would somewhat offset each other but the net impact might be that the octane rating decreases.

Don't get excited though by seeing the higher octane number; volatile light components are what goes first, and while the portion left behind has higher density/higher octane, it's unavailable for cold starts/high revving, etc.


Is this for an open container? These changes seem large for gas in a closed gas tank.

 

[SonofJoe]

If you look at engine oil fuel dilution, the stuff that you find mixed in with your oil is more like very heavy naphtha/very light kerosene than the gasoline you put in your tank. This the stuff that doesn't instantly evaporate when you mix the fuel with the air and works it's way past the rings into the sump. In this context, all talk of octane is sort of meaningless.
One thing you will find us that while a long drive on the motorway will drive some of the dilution out if the oil, once mixed, it is nigh on impossible to separate ALL of the gas out of the oil. It's just how the physics play out...