Can a slight change in Oil Viscosity Affect Your Mileage? - Smart Car Forums
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#1 (permalink) Old 01-08-2011, 02:46 PM
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Can a slight change in Oil Viscosity Affect Your Mileage?

About 6 months ago I installed an Autometer "ecometer" and programmed it for my Smart. The ecometer has been reading my miles per gal at 42MPG. Recently, I changed my oil from 3.5 Qts of Mobile1 5W30 to 2.5 Qts of Mobile1 10W30 and 1QT of Lucas Oil Synthetic additive. I figured the additive was warranted since I am hard on the engine. I like to run the engine in the 3K to 5K range between shifts. Now my mileage is down to 39 MPG. Does anyone know if a slight change in oil viscosity can affect mileage this drastically? The only change was the oil. I still drive like a nut just like I used to before the drop in MPG. Any thoughts?

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#2 (permalink) Old 01-08-2011, 03:05 PM
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Winter blend gasoline laced with ethanol could cause your mileage to drop. Oil viscosity can have an effect as well. Why not use 0w/40 synthetic oil and skip the additive?
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#3 (permalink) Old 01-08-2011, 03:13 PM
 
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Its winter gas blend. My mileage drops 3-4mpg per season
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#4 (permalink) Old 01-08-2011, 03:17 PM
 
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My friends ford focus uses 5w 20 synthetic... It is what ford recommends... Now thats thin for year round...
~A Must Read To The END~
http://www.machinerylubrication.com/Read/518/motor-oils
SAE 5W-20

Last edited by Brabus007; 01-08-2011 at 04:12 PM.
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#5 (permalink) Old 01-08-2011, 03:39 PM
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12 Months of MPG records for my smart.

It appears to me, that at least in my case, Air Conditioning use is a factor which overwhelms any effect of a Winter Blend.
Is this winter mix used in all regions?

Is it the consensus that motor engine oil viscosity has no effect on MPG? That seems counter intuitive to me.
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#6 (permalink) Old 01-08-2011, 04:13 PM
 
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Oil viscosity can have a huge effect sometimes, but it varies tremendously from car to car.

Has anyone here even tried stuff with this car yet? Smart seems very specific about wanting 0W40, but I refuse to believe that's warranted at all.

How long are your trips in general? If your drive length gives the car plenty of time to warm up, then there shouldn't be much difference between 5W30 and 10W30. If you make lots of short trips, so the oil is cold most of the time, then a much bigger difference is to be expected.

See Bob is the Oil Guy for much more information on motor oil and its operation than you ever wanted to know.

I'm trying some 0W30 at the next oil change (which is very soon).
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#7 (permalink) Old 01-08-2011, 04:27 PM
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Although winter blend fuel could get a lot of the blame, viscosity can affect MPG. That's why Mobil1 and others produce their "Advanced Fuel Economy" variations. I've been running 0W-30 AFE for a while now.

In this case, the 10W (Winter weight) could be a contributing factor in the lower MPG (especially for short trips) and would also increase engine wear on cold starts. The oil will be thicker when cold and probably take longer to get up to operating temp. However, once up to operating temperature, there shouldn't be any noticeable MPG difference between a 5W-30 and a 10W-30. If you do mostly longer trips, then it shouldn't make much of a difference in MPG.

Regardless, unless you live in Puerto Rico, I would definitely switch to 0W for cold weather engine protection.
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#8 (permalink) Old 01-08-2011, 04:44 PM
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Cold weather trip lengths

I had not thought of the lengths of my trips. Now that you all mention it, my trips this winter have been more local lately. So, if my oil is taking longer to heat up to proper viscosity I am in effect reducing my ability to reach optimal mpg. At my next oil change I need to go back to 5W30 Mobile1 and run it all year round. Thanks!

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#9 (permalink) Old 01-08-2011, 05:13 PM
 
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Quote:
Originally Posted by Brabus007 View Post
My friends ford focus uses 5w 20 synthetic... It is what ford recommends... Now thats thin for year round...
~A Must Read To The END~
Motor Oils - Fuel Economy vs. Wear
SAE 5W-20
The second link doesn't look trustworthy to me because they are trying to sell their own product.

The first link does have a valid point, but only if you're always running your engine wrong somehow or another. They are talking about the oil film, which does get generally better with higher viscosity. If you're doing it right, then you're not relying on the film for separation. The separation forces other than the film are defined by oil flow, not pressure. Oil pumps for cars are constant flow (per RPM) devices.

At high RPM, the dominant forces are from the reciprocating motion of parts. The oil flow increases with the RPM to match the higher forces those parts produce. If the oil is too thick, however, it may provide high enough pressure across the pump to activate the pressure-limiting bypass valve. Revving faster than that will increase the forces that need to be controlled, but not the oil flow that's supposed to be doing the controlling. Using a thicker oil will keep you from doing so much damage when you get into that mode of operation, but if you had used a thinner oil you may not have gotten that problem in the first place.

At low RPM, the dominant forces are from the events in the cylinders. These get bigger with higher load. The flow will be the same no matter what the viscosity of your oil, so thicker oil will lubricate better (better film). This is why low-RPM high-load operation is considered bad.

Simple version: thinner oil works better at higher revs, thicker oil works better at lower revs, if you know what you're doing so as not to kill your engine you can get better MPG with thinner oil.

Also, keep in mind the effects of temperature on viscosity.
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#10 (permalink) Old 01-08-2011, 05:26 PM
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Do not forget the car runs very rich until it hits running temp.
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