Fuel economy of oil not so simple?

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Alright so ive been searching these forums for the answer to this and I can't seem to find it.

I keep reading that 0w20 is more fuel efficient that 5w20 and when I was new here that made sense to me (thinner on start up) but I just bought some Pennzoil Ultra 5w20 and wanted to use it in my prius spec'd for 0w20...so I started looking to see how much it would reduce my fuel.

As far as I know, you would base fuel economy on both the 40 degree C viscosity and the 100 C viscosity. Where I live the car gets started at more like 35-65 (F) so I am not sure how to factor that in. But the 100C temp should be consistent between 0 and 5 w20. So I assume this means 0w20 is more efficient for shorter trips.

BUT!!! I started looking more on the viscosity numbers at 40C and 100C and this is what I noticed.

M1 AFE 0w20----@40C:44.8-----@100C:8.7
PU 5w20--------@40C:48-------@100C:8.8
Amsoil 0w20----@40C:46.6-----@100C:8.7

As far as I can tell, at normal temperatures there is very little to gain from the M1 0w20

Any insight on this issue?

Also when you compare to a 0w30 or 0w40 oil too I notice (dont have data sheets handy) that the start up @40C is SIGNIFICANTLY higher than the 5w20. I used 0w40 penn euro thinking I was keeping the startup wear minimized...
 
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HT/HS viscosity impacts fuel economy more than kinematic viscosity, but even then, I wouldn't expect any significant differences.
 
Also I wonder what the average oil temperature would be in a Prius? Say 70F outside?
Probably not near 100C at any time(unless it has a heat exchanger with the engine coolant?), and maybe a lot lower in city traffic when the electric motor is doing more.
Oil temps also vary alot with engine rpm and not not so much load, and I assume a prius runs pretty low rpms in normal driving.
 
The 40C (which, btw, is around 100F, so not cold at all) number isn't relevant to the "W" SAE rating of the oil. That's why 0W40 has a much higher kinematic viscosity at 40C than 0W20. Here are the relevant numbers.
Code:


SAE Cranking Pumping Viscosity

Viscosity Viscosity (mPa*s)

Grade (mPa*s) at temp at temp

0W 6200 at -35 60000 at -40

5W 6600 at -30 60000 at -35

10W 7000 at -25 60000 at -30

15W 7000 at -20 60000 at -25

20W 9500 at -15 60000 at -20

25W 13000 at -10 60000 at -15


source: http://www.viscopedia.com/viscosity-tables/substances/sae-viscosity-grades/
 
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40C is 104F, so if your "typical" start-up temps are 2-18 degrees C, I'm not sure why focusing on the 40C viscosity is helping you?

The 0w-xx designation denotes superior performance at low temperature extremes. MRV is measured at -40C, CCS at -35C for any oils wearing the 0w-xx label.

Not all 0w-xx oils are extremely high VI (viscosity index, which is the difference between the 40C value and the 100C value) and an oil blended with a ton of PAO (one of the best base oils for extremely good cold temperature performance) and very little VII (Viscosity Index Improver) will fall into that category.

Generally, for a given grade, a 0w-xx version will have a higher VI (less viscosity change with temperature) than a 5w-xx counterpart, however that is not always the case. Overall, the effect on fuel consumption is extremely small, so small, that it needs to be really measured in a lab environment. Chasing fuel economy gains between 0w-20 and 5w-20 is an exercise in futility. The real gains will be improved cold temperature performance (cold cranking) in places that see extreme cold. That's where you may actually see a difference between the two. Winnipeg or Edmonton come to mind, or any place in Alaska.
 
I was focusing on those nmbers because those are what I had access to and it made sense to me, I am not fully informed on all oil topics and wanted to explore fuel economy a bit more. What numbers on the data sheets should I focus on?
 
Originally Posted By: TheKracken
What numbers on the data sheets should I focus on?

I would stop worrying about it. Any differences in fuel economy between 0w-20 and 5w-20 oils will be negligible in your climate.
 
If the fact that the AFE is less viscous at 40C holds true to reasonable ambient (i.e. say near freezing to 70F) "cold start" temperatures, then it may offer some benefit there. IIRC, one of the big reasons to go 5w-30 vs 10w-30 or -40 years ago, was slightly improved MPG numbers at cold start. Such a benefit might be in the mix here.

AFE might also have a higher level of friction modification as well...
 
Originally Posted By: IndyIan

Oil temps also vary alot with engine rpm and not so much load, and I assume a prius runs pretty low rpms in normal driving.


This may be true for some engines, such as the Prius here, but will vary greatly with engine design and is not a good rule of thumb. For example, engines that have piston cooling nozzles will see the oil pan temp track very closely to fuel usage or exhaust gas temps.
 
Originally Posted By: OVERKILL
Chasing fuel economy gains between 0w-20 and 5w-20 is an exercise in futility.


I'd like to see if there are any differences between 0W-20 and 0W-30.....

or between 5W-20 and 5W-30.

I'll bet not much.
 
Every once in a while I check mpg, and I have the EVIC which displays mpg. Switching from 0W20 to 5W30 made no difference that I could measure, or see a change displayed by the EVIC in my wife's Liberty.
 
Originally Posted By: JHZR2
If the fact that the AFE is less viscous at 40C holds true to reasonable ambient (i.e. say near freezing to 70F) "cold start" temperatures, then it may offer some benefit there. IIRC, one of the big reasons to go 5w-30 vs 10w-30 or -40 years ago, was slightly improved MPG numbers at cold start. Such a benefit might be in the mix here.

AFE might also have a higher level of friction modification as well...


Drat, I was going to mention friction modifiers.
And I was going to mention HTHS effect on fuel economy.
M1 AFE is probably the best oil out there that maximizes the fuel economy tricks that are available to oil blenders.
But don't expect to put your kids through college on the fuel savings.
To get any more significant improvement in fuel economy via an oil change would be to put in a 0w16 oil that is down around 2.3 HTHS. And I don't think your engine is back-spec'd for it.
 
Originally Posted By: 09_GXP
Originally Posted By: IndyIan

Oil temps also vary alot with engine rpm and not so much load, and I assume a prius runs pretty low rpms in normal driving.


This may be true for some engines, such as the Prius here, but will vary greatly with engine design and is not a good rule of thumb. For example, engines that have piston cooling nozzles will see the oil pan temp track very closely to fuel usage or exhaust gas temps.


All the engines I've had with piston cooling nozzles still had the RPM and oil temp in sync.
 
Reading all posted answers was interesting. Here is my info you should know...
Fuel savings between the 2 grades can only be measured in lab since it is so small( you would not be able to measure it in any fuel savings!). Stay with the factory Spec. 0w-20 for best life of vehicle{Start-up is were the majority of your wear is.} as well as your warranty. Going even into the desert I would stay with what they recommend, since normal driving habits would not do any damage to the engine even under 104 degrees[That was designed into the engine by the manufacturer already.]!
Final note: Never use any oil additives that are being advertised on your local car parts stores! This will not only do no improvements( I know since I tried most of them over the years[Total waste of my money!
cry.gif
],but will surely void any factory warranties(they will check for this kind of thing when you start having any issues.).
 
KV viscosity is of little significance in fuel economy and the W numbers are only relevant in the kind of extreme cold you'll never see in Seattle.
HTHSv is what matters, as well as VI. A higher HTHSv will yield lower fuel economy while a higher VI will likely yield better fuel, since the two are negatively correlated.
Achieving a high VI in a finished oil requires heavy VII treat rates, which results in lower HTHSv due to temporary shear relative to its KV viscosity.
Friction modifiers also enter into this, but if your goal is no more than maximum fuel economy without any concerns about VII treat along with a healthy dose of FM moly, something like TGMO might be a good pick.
After a number of years here, it wouldn't be mine.
 
I posted this first on Brother Gokhan's recent 'Thick vs Thin' troll-fest but it might be more usefully reposted here as it's to do with fuel economy...


Just thought I'd add something else based on the numbers in SR5's Valvoline table (if some kind person could cut and paste the table into this thread, it would be very helpful) ...

Most folks 'get' why low viscosity oils might potentially be good for fuel economy. However what they may not realise is there are two parts to this arguement and one isn't always obvious.

In the Valvoline table, it shows a 10W30 and a 5W30 both with nominally the same KV100 (10.9 & 11.0 cst). Once these oils are up to operational temperature (for the sake of argument, say that's 100°C), both oils will give essentially the same fuel economy.

That the 5W30 gives better FE than a 10W30 isn't always obvious because the W rating is generally considered to be all about low temperature startability. The 10W30 is designed to pump at -30°C (-22°F) and crank at -25°C (-13°F). A 5W30 is designed to go even lower at -35°C (pump) and -30°C (crank). [actually both the Valvoline oil should work a few °C below the formal limits]. So on the surface, the W rating has naff all to do with better fuel economy!

However for a fixed KV100 (or oil weight), dropping the W rating has the side effect of dropping the oil's KV40. In the Valvoline table, in moving from the 10W30 to the 5W30, the KV40 of the oil drops from 70.5 cst to 62.8 cst. If you go even lower to a 'typical' start-up temperature (say 10°C for the UK), the 10W30 has a KV10 of 351.5 cst while the 5W30 has a KV10 of 271.7 cst. Now a 30% higher oil KV does not worsen fuel economy by 30%! I've forgotten most of the fluid dynamics I learnt at uni but I do recall that viscosity is a minor factor in Reynolds Number and Prandtl Number. However lower oil viscosity, as the oil warms-up from say 10°C to 100°C, should yield a couple percent fuel economy benefit.

Now if you contrast the 5W30 with the 5W20 in the Valvoline table, not only does the KV100 drop (from 11.0 cst to 8.6 cst) but the KV40 also drops (from 62.8 cst to 48.9 cst). The 'start-up' KV10s also drop (from 271.7 and 217.5 cst).

So here's the thing.... if you drop an oil weight (but not the W rating) you get two fuel economy savings; one during start-up/warm-up and one when your oil's up to operational temperature. If you just drop a W rating (but keep the weight the same) you only get one saving (during start-up/warm-up).

Now contrast the 5W20 with the 0W20 in the Valvoline table. The KV100's are nominally the same (8.6 vs 8.7), the KV40 drops but not by much (48.9 to 45.2) while the calculated KV10 drops from 217.5 to 181.2). This is very interesting. Given the way thermo-dynamics works, the RATE at which the temperature of your oil increases should be fastest between 10°C and 40°C, slower between 40°C and 70°C and slower still between 70°C and 100°C (after which it notionally reaches steady state). If this is the case, on an time-aggregated basis, I would not expect the 0W20 to give much of a fuel economy benefit over and above what you see with the 5W20 during the warm-up phase and no benefit at operational temperature.

So in relative terms, oil weight probably trumps W-rating in delivering tangible fuel economy.

Which is interesting because lower W-rating usually comes with more problems; more expensive base oil, more VII, more Ashless and higher Noack). Just dropping an oil weight does exactly the reverse.

So 10W20? Wherefore art thou??
 
Originally Posted By: SonofJoe
I posted this first on Brother Gokhan's recent 'Thick vs Thin' troll-fest but it might be more usefully reposted here as it's to do with fuel economy...


Just thought I'd add something else based on the numbers in SR5's Valvoline table (if some kind person could cut and paste the table into this thread, it would be very helpful) ...

Most folks 'get' why low viscosity oils might potentially be good for fuel economy. However what they may not realise is there are two parts to this arguement and one isn't always obvious.

In the Valvoline table, it shows a 10W30 and a 5W30 both with nominally the same KV100 (10.9 & 11.0 cst). Once these oils are up to operational temperature (for the sake of argument, say that's 100°C), both oils will give essentially the same fuel economy.

That the 5W30 gives better FE than a 10W30 isn't always obvious because the W rating is generally considered to be all about low temperature startability. The 10W30 is designed to pump at -30°C (-22°F) and crank at -25°C (-13°F). A 5W30 is designed to go even lower at -35°C (pump) and -30°C (crank). [actually both the Valvoline oil should work a few °C below the formal limits]. So on the surface, the W rating has naff all to do with better fuel economy!

However for a fixed KV100 (or oil weight), dropping the W rating has the side effect of dropping the oil's KV40. In the Valvoline table, in moving from the 10W30 to the 5W30, the KV40 of the oil drops from 70.5 cst to 62.8 cst. If you go even lower to a 'typical' start-up temperature (say 10°C for the UK), the 10W30 has a KV10 of 351.5 cst while the 5W30 has a KV10 of 271.7 cst. Now a 30% higher oil KV does not worsen fuel economy by 30%! I've forgotten most of the fluid dynamics I learnt at uni but I do recall that viscosity is a minor factor in Reynolds Number and Prandtl Number. However lower oil viscosity, as the oil warms-up from say 10°C to 100°C, should yield a couple percent fuel economy benefit.

Now if you contrast the 5W30 with the 5W20 in the Valvoline table, not only does the KV100 drop (from 11.0 cst to 8.6 cst) but the KV40 also drops (from 62.8 cst to 48.9 cst). The 'start-up' KV10s also drop (from 271.7 and 217.5 cst).

So here's the thing.... if you drop an oil weight (but not the W rating) you get two fuel economy savings; one during start-up/warm-up and one when your oil's up to operational temperature. If you just drop a W rating (but keep the weight the same) you only get one saving (during start-up/warm-up).

Now contrast the 5W20 with the 0W20 in the Valvoline table. The KV100's are nominally the same (8.6 vs 8.7), the KV40 drops but not by much (48.9 to 45.2) while the calculated KV10 drops from 217.5 to 181.2). This is very interesting. Given the way thermo-dynamics works, the RATE at which the temperature of your oil increases should be fastest between 10°C and 40°C, slower between 40°C and 70°C and slower still between 70°C and 100°C (after which it notionally reaches steady state). If this is the case, on an time-aggregated basis, I would not expect the 0W20 to give much of a fuel economy benefit over and above what you see with the 5W20 during the warm-up phase and no benefit at operational temperature.

So in relative terms, oil weight probably trumps W-rating in delivering tangible fuel economy.

Which is interesting because lower W-rating usually comes with more problems; more expensive base oil, more VII, more Ashless and higher Noack). Just dropping an oil weight does exactly the reverse.

So 10W20? Wherefore art thou??
Originally Posted By: SonofJoe
I posted this first on Brother Gokhan's recent 'Thick vs Thin' troll-fest but it might be more usefully reposted here as it's to do with fuel economy...


Just thought I'd add something else based on the numbers in SR5's Valvoline table (if some kind person could cut and paste the table into this thread, it would be very helpful) ...

Most folks 'get' why low viscosity oils might potentially be good for fuel economy. However what they may not realise is there are two parts to this arguement and one isn't always obvious.

In the Valvoline table, it shows a 10W30 and a 5W30 both with nominally the same KV100 (10.9 & 11.0 cst). Once these oils are up to operational temperature (for the sake of argument, say that's 100°C), both oils will give essentially the same fuel economy.

That the 5W30 gives better FE than a 10W30 isn't always obvious because the W rating is generally considered to be all about low temperature startability. The 10W30 is designed to pump at -30°C (-22°F) and crank at -25°C (-13°F). A 5W30 is designed to go even lower at -35°C (pump) and -30°C (crank). [actually both the Valvoline oil should work a few °C below the formal limits]. So on the surface, the W rating has naff all to do with better fuel economy!

However for a fixed KV100 (or oil weight), dropping the W rating has the side effect of dropping the oil's KV40. In the Valvoline table, in moving from the 10W30 to the 5W30, the KV40 of the oil drops from 70.5 cst to 62.8 cst. If you go even lower to a 'typical' start-up temperature (say 10°C for the UK), the 10W30 has a KV10 of 351.5 cst while the 5W30 has a KV10 of 271.7 cst. Now a 30% higher oil KV does not worsen fuel economy by 30%! I've forgotten most of the fluid dynamics I learnt at uni but I do recall that viscosity is a minor factor in Reynolds Number and Prandtl Number. However lower oil viscosity, as the oil warms-up from say 10°C to 100°C, should yield a couple percent fuel economy benefit.

Now if you contrast the 5W30 with the 5W20 in the Valvoline table, not only does the KV100 drop (from 11.0 cst to 8.6 cst) but the KV40 also drops (from 62.8 cst to 48.9 cst). The 'start-up' KV10s also drop (from 271.7 and 217.5 cst).

So here's the thing.... if you drop an oil weight (but not the W rating) you get two fuel economy savings; one during start-up/warm-up and one when your oil's up to operational temperature. If you just drop a W rating (but keep the weight the same) you only get one saving (during start-up/warm-up).

Now contrast the 5W20 with the 0W20 in the Valvoline table. The KV100's are nominally the same (8.6 vs 8.7), the KV40 drops but not by much (48.9 to 45.2) while the calculated KV10 drops from 217.5 to 181.2). This is very interesting. Given the way thermo-dynamics works, the RATE at which the temperature of your oil increases should be fastest between 10°C and 40°C, slower between 40°C and 70°C and slower still between 70°C and 100°C (after which it notionally reaches steady state). If this is the case, on an time-aggregated basis, I would not expect the 0W20 to give much of a fuel economy benefit over and above what you see with the 5W20 during the warm-up phase and no benefit at operational temperature.

So in relative terms, oil weight probably trumps W-rating in delivering tangible fuel economy.

Which is interesting because lower W-rating usually comes with more problems; more expensive base oil, more VII, more Ashless and higher Noack). Just dropping an oil weight does exactly the reverse.

So 10W20? Wherefore art thou??


10w20= sae20?
 
Yes it's quite possible that an SAE 20 is a 10W20 but that's sort of beside the point if it's not labelled as such.

The average bloke in the street doesn't know much about oil but they do generally get the thing about multigrades being suitable for both winter and summer. For this reason he would expect to see a W-rating on the can. Some of the old guys might remember back to the days when 10W30 reigned supreme in the US and make the link that if 10W was acceptable then, maybe it's still acceptable today. They may also have heard that 20-weight oils are not beyond the pale today (although it's clear for some they are). Put the two things together and you have the potential for making a fuel economy oil without the 'bad' bits.

Simples...
 
It's funny, but your monograde 10W20 would warm up quicker than an uber VI 0W20, with more viscous drag in that warmup regime.

Looking at Toyota and Honda stuff, it's the warm-up range that they are really targetting, short tripping.

Do you think that you could make a mono 20 with an HTHS around the 2.6 mark instead of 2.8 to 3 ?
warmup.jpg
 
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