Thinnest possible oil you can put in a car?

OVERKILL

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I learned it in the 60s from an engine builder in Cleveland who did flathead Fords for stock cars. So it goes back many decades. And with oil formulas and metallurgy so much better now, I have to think it's still a good number.

It's just a guideline for determining visc requirements based on clearances selected in performance builds, it shouldn't be used to discern minimum safe viscosity for an already engineered product.

Say you built a 355 CASCAR mill and went a bit generous on the bearing clearances. It takes 15w-40 to get you to 20psi at 2,000RPM, clearly you aren't going to be running 10w-30 in it.

That's not the same as say Porsche developing an engine that's intended to use a lubricant carrying A40, which requires extensive testing, has minimum HTHS requirements, AW requirements...etc. You can't just conclude that since that engine produces 30psi at 800rpm on 0w-40 that you can run 0w-8 in it.
 
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KY, USA
The two best things to reduce fuel consumption are driving like you don't have a brake pedal and slowing down. My '16 Versa is EPA rated at 31/city, 39/highway and 34/combined. If I reduce my speed to about 50 MPH, use the brakes minimally, and try to time traffic lights so I'm not always stopping for them and sitting idle in the summertime I can often get 50+ MPG combined. Reducing speed even further helps even more. Most of my driving in the Versa is on 2/4 lane roads in the 40-55 MPH range and since purchase in Feb. 2019 with 15947 miles tracked my overall MPG average is 48.874 MPG. I'm also using 10w40 year around in the Versa.

For years I drove an '88 Ford Escort Pony as a daily driver and also used 10w40 in it. I could drive it 80-90 MPH on the highway and still get 40+ MPG. If the mileage ever dropped below 40 MPG in it it was because I was starting it up early to let it warm up in winter time or there was something wrong with and the car needed attention. When I retired it it had 518K miles and was still delivering over 40 MPG.

In almost 45 years worth of driving and car ownership I don't think I've ever had a car that I couldn't beat EPA rating by several percent.
 
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378
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Scottsdale, AZ
I’ve gone up a grade in a qr25de mostly out of convenience (5w30 instead of 0w20 and will be moving it to 0w40 once I get through my 5w30) practically had zero impact on fuel economy using a standard drive we do from AZ to CO using the odometer based method of fuel economy calculation. Right foot, engine braking, and judicious use of cruise control in my experience is going to have a much bigger impact than oil type. As far as tire pressure, isn’t that a false economy? Seems like you’d burn through tires faster costing more money than the .05 mpg would cost. Also are we sure that overinflation is more efficient on bumpier roads from a rolling resistance perspective.
 
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As far as tire pressure, isn’t that a false economy? Seems like you’d burn through tires faster costing more money than the .05 mpg would cost. Also are we sure that overinflation is more efficient on bumpier roads from a rolling resistance perspective.


No when the term “properly inflated” is used. A lot of drivers are running on under inflated tires due to ignorance or laziness.
 
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I believe that old "rule of thumb" was specifically geared toward the SBC and was just a general guideline, I would certainly not be using it as a validation mechanism for deviating significantly from spec. Honda went with wider bearings and journals for example when they started going thinner, and this was discussed on here at some length a while back as the Japanese OEM's were the first to really chase the ultra-thin oil thing.

I would never want the following exchange to play-out as a result of advice received here:

Bob: Gee Fred, how'd you toss that rod through the side of the block?
Fred: Well you know Bob, I was on this message board and I was told as long as 0w-16 gave me 10psi per 1,000RPM, I'd be fine, so I put it in and I had 10psi at 1,000RPM so off I went. Engine started knocking after I ran it hard up an on-ramp and then we had a big 'ol cloud of smoke and here we are!
Bob: Doesn't your engine spec like 5w-40?
Fred: Yes, but these boys were really convincing, they really seemed know their stuff!
Bob: Are they going to be paying to fix this?

*crickets*
🤣🤣
 
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California
Don't change the oil grade just for fuel economy. The gains are less than 1% and your brother will see much better gains by following the simple "hypermiler" suggestions in this thread, such as higher tire pressure, anticipate red lights, don't accelerate hard, don't drive too fast, take out extra weight, etc etc. Also, old car doesn't equal bad fuel economy. My 1967 Mustang 6-cylinder got 24 mpg with only 50% highway miles, clocked this number more than once. So with 110hp from a 1.6L 16 valve engine, performance-wise that is like a Corolla or Civic or other economy car. So I will guess that your brother's car should be pretty good on gas already... unless it's worn out. But anyway, thinner oil is not the place to look for better fuel economy. Tell him "don't do it."
 
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Did everyone in the replies not read the post? The owner of the vehicle is willing to try some wild stuff.. so I say go for it.

0w2 oil exists for drag racing engines.. so I would assume it can be found in Europe or wherever the OP lives.


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PNW
Didn't know sae had 0W2
Is that a typo?
If 0w2 isn't at 200F when it's used in a drag racing engine, then it's still thicker than one would expect. Racers use super thin oils like this because they can't heat up the oil to full operating temperature before the run down the drag strip. If the 0W2 is only 100F, then it's probably more like a 5W-40 at 212F.
 
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Paramount, California
Its a 2004 1.6 16v gasoline engine with 110hp and 5 speed manual trans, oem spec oil is 5w40.
That's a very low-BMEP engine. You don't need SAE xW-40 in such a low-output engine. SAE xW-30 is more than thick enough, and even SAE xW-20 is fine if there is no oil consumption. I find it hard to believe that the OEM spec'd SAE xW-40 unless it is a diesel with high torque.
 

OVERKILL

$100 Site Donor 2021
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Did everyone in the replies not read the post? The owner of the vehicle is willing to try some wild stuff.. so I say go for it.

0w2 oil exists for drag racing engines.. so I would assume it can be found in Europe or wherever the OP lives.


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There's no such grade, which is why it's 0w2 not 0w-2
 
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