Thinnest possible oil you can put in a car?

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186
Location
The Netherlands
My brother has an old car and currently has a 120 mile daily highway drive.
He asked me what i could do to make it absolutly as fuel efficient as possible.
I said it probably wont make a lot of difference and maybe try a simple thing like putting a little more pressure in the tyres.
And if you want to make it extreme take a thinner engine oil and trans oil but there are hazzards attached to doing this and i wont recomend it for the very smal gains.

He however wants to try it so i looked into some oils.

Its a 2004 1.6 16v gasoline engine with 110hp and 5 speed manual trans, oem spec oil is 5w40.
Currently theres motul 5w40 in it which has a the following specs:

Viscosity grade SAE J 300 5W-40
Density at 20°C (68°F) ASTM D1298 0.851
Viscosity at 40°C (104°F) ASTM D445 85.4 mm²/s
Viscosity at 100°C (212°F) ASTM D445 14.2 mm²/s
HTHS viscosity at 150°C (302°F) ASTM D4741 3.7 mPa.s
Viscosity Index ASTM D2270 172
Pour point ASTM D97 -36°C / -33°F
Flash point ASTM D92 230°C / 450°F
Sulfated ash ASTM D874 1.1% weight
TBN ASTM D2896 10.1 mg KOH/g

I looked into ravenol 5w20 SFE which has the following specs, i liked it because i think its a pretty ''thick'' 5w20 and has a HTHS of 2.9

Density at 20°Ckg/m³843,0EN ISO 12185
Colourbraunvisual
Viscosity at 100°Cmm²/s8,5DIN 51 562
Viscosity at 40°Cmm²/s47,2DIN 51 562
Viscosity index VI160DIN ISO 2909
HTHS at 150°CmPа*s2,9CEC L-036-90
CCS Viscosity at -30°CmPa*s3640ASTM D5293
Low Temp. Pumping viscosity (MRV) at -35°CmPa*s6.700ASTM D4684
Pourpoint°C-63DIN ISO 3016
Noack Volatility% M/M8,3ASTM D5800/b
Flash point°C238DIN ISO 2592
TBNmg KOH/g8,8ASTM D2896
Sulphated ash%wt.1,07DIN 51 575


These engines are known to be pretty bulletproof , i used to own one and did a lot of 120mph prolonged highway runs where oil temps would reach 285f 140c, that was also on 5w40.


Does anybody have experience with switching to thinner oils? Did you encounter problems with this? Normally you can pretty safely switch from 5w40 to 5w30, however from 5w40 to 5w20 is a large step.

Basicly as long as the oil pressure is stable and high enough i dont forsee any real short term problems, except maybe some more oil consumption. (normaly these engines use absolutly 0)

I told him its a pretty large risk to take suchs large steps for almost inmeasureable amount of fuel consumption reduction. But he wants to try it.

You can get these engines for around 100 euros at local junkyards so its not that big of a risk. As i said they normally never fail so they are not that sought after.

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Messages
5,538
Location
down in the park
Fit a full body undertray to keep airflow attached, Remove as much as possible the toe and camber from the wheels, get anything that isn't absolutely necessary out of the car to save weight, including putting your brother on a diet. Use shutters or blind off parts of the radiator... Drive slower... But at 120 miles a day, he'd be better off with a diesel engine.

There's a lot you can do but the question is will it reduce fuel consumption enough to offset the costs?
 
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17,243
Location
...
There are a lot of other things he can do to increase fuel efficiency. Properly inflated tires, keep a light foot on the throttle and cruise to lights and stops and much more.

I would only run the oil that the manual recommended.
 

OVERKILL

$100 Site Donor 2021
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46,081
Location
Ontario, Canada
The gains realized from a change in viscosity will be extremely small, your suggestion to bump up tire pressure will be far more effective. Fuel economy gained from a drop in viscosity is spread across total sales volume, at which point it becomes significant. It's typically not even statistically measurable by "average Joe". Mobil claims up to 2% going from a heavier grade to their AFE products:

Mobil said:
Mobil 1 0W‐20 and 0W‐30 Advanced Fuel Economy synthetic motor oils deliver up to 2 percent fuel economy improvement* and can save drivers more than $400 on gasoline over the life of their vehicles. That's a savings of about six cents per gallon of fuel. A typical passenger car with a 20‐gallon gas tank can go up to nine more miles on a tank of gas when using Mobil 1™ Advanced Fuel Economy in either 0W-20 or 0W-30. That equates to up to 250 more miles per year under typical conditions.

*Fuel economy improvement is based on a comparison versus those viscosity grades most commonly used and a potential 2 percent fuel economy improvement. Actual savings are dependent upon vehicle/engine type, outside temperature, driving conditions and your current engine oil viscosity. Savings estimates are based on a gasoline cost of $3 per gallon, average fuel economy of 22.5 miles per gallon, annual mileage of 12,500 and lifetime mileage of 150,000.

That's a 0.45mpg gain at the most.
 

Flyingdutchman

Thread starter
Messages
186
Location
The Netherlands
The gains realized from a change in viscosity will be extremely small, your suggestion to bump up tire pressure will be far more effective. Fuel economy gained from a drop in viscosity is spread across total sales volume, at which point it becomes significant. It's typically not even statistically measurable by "average Joe". Mobil claims up to 2% going from a heavier grade to their AFE products:
Yeah told him that, i tried hypermiling my car once. The savings where also almost unmeasurable and i like to drive fast so it was no fun.
Internal combustion engine is just a highly inneficient process :ROFLMAO:

Will probably inlfate the tyres and thats it. The driving style is up to him.

Pouring 80 euros of oil into it will never repay itself back.
 
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4,709
Yeah told him that, i tried hypermiling my car once. The savings where also almost unmeasurable and i like to drive fast so it was no fun.
Internal combustion engine is just a highly inneficient process :ROFLMAO:

Will probably inlfate the tyres and thats it. The driving style is up to him.

Pouring 80 euros of oil into it will never repay itself back.

I have been told inflating the tires on the high side doesn't yield the results people think it does...
 

Flyingdutchman

Thread starter
Messages
186
Location
The Netherlands
are there other oils in the manual with 2.9 cP hths?
The thinnest you can go is 5w30 a3 b3 in colder climate according to the manual. I have plenty of BMW ll04 5w30 laying around(because my car consumes so much :ROFLMAO: ).. It has a hths of 3.5.

Also the oem trans oil is already 75w80 so no real point in going thinner.
I have been told inflating the tires on the high side doesn't yield the results people think it does...
My experience is that it does save some but the wear pattern, the ride quality, and the handeling is ruined.
 
Messages
35,696
Location
NY
As already mentioned stick with the spec'd oil, take junk out of the trunk, and increase tire pressure to the maximum safe pressure they can handle. I seriously doubt your brother will be able to calculate the savings thinner oil will produce in increased mpg, especially in an old car. Have him drive with fuel economy in mind. Sometimes something as simple as that can yield the best gains.
 

Flyingdutchman

Thread starter
Messages
186
Location
The Netherlands
As already mentioned stick with the spec'd oil, take junk out of the trunk, and increase tire pressure to the maximum safe pressure they can handle. I seriously doubt your brother will be able to calculate the savings thinner oil will produce in increased mpg, especially in an old car. Have him drive with fuel economy in mind. Sometimes something as simple as that can yield the best gains.
Yeah its verry dificult to improve fuel consumption, a 1986 toyota starlet 1.6 has about the same fuel consumption as a new 2020 golf 8 speed DSG 1.2 150hp turbo 3 cylinder.

According to the specs the golf is mega efficient. In reality its almost the same, you just need X amount of fuel to create X amount of energy to travel.
The thinnest I would go is 5w30 in A3/B4 or C3....
Yeah i have a lot of 5w30 laying around, at the next oil change i will put that in it.

My dad put very thin oil in an older car once, not remeber the exact specs anymore but it was way too thin.
The engine did not fail or run verry different, but when you let it idle for example at a stoplight you could see it smoking out of the exhaust. The oil was so thin that it creeped past the rings.
 
Messages
856
Location
Daytona Beach
Just tell him this "Every time you touch the brakes, you wasted fuel before that".
I can't drive like that, but maybe he can.
Of course, tires being a couple pounds over door jamb numbers will help the most.
 
Messages
10,890
Location
Cincinnati, OH, USA
Not sure what kinds of roads he drives on, but keeping overall speed down can yield a sizable fuel economy gain. For instance, flat back roads 55 MPH (around 88-89 KPH) gives 25 MPG in the heavy (but aerodynamic) Mercury in my sig-blasting down the interstate at 75-80 (130 KPH) drops MPG to under 20!
 

Flyingdutchman

Thread starter
Messages
186
Location
The Netherlands
Just tell him this "Every time you touch the brakes, you wasted fuel before that".
I can't drive like that, but maybe he can.
Of course, tires being a couple pounds over door jamb numbers will help the most.
Not sure what kinds of roads he drives on, but keeping overall speed down can yield a sizable fuel economy gain. For instance, flat back roads 55 MPH (around 88-89 KPH) gives 25 MPG in the heavy (but aerodynamic) Mercury in my sig-blasting down the interstate at 75-80 (130 KPH) drops MPG to under 20!
yeah in our family we also have a issue with driving slowly so i dont see that happening :ROFLMAO: , would be the best though.
 
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