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  #1  
Old 06-22-2010, 06:13 PM
Rigamarole Rigamarole is offline
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What's the Straight Dope on 5W-20 vs. 5W-30 (or other viscosity) motor oil?

I have a fairly new (model year 2007) Mazda and the manual recommends 5W-20 oil, which is what I have been using to this point. But I did some Google searches to educate myself about oil and there seems to be a point of contention over whether this is the "best" oil to use. I'm finding a lot of conflicting and unsubstantiated opinions on this so I wanted to turn it to you guys.

Some people say that car manufacturers started putting 5W-20 recommendations into manuals a few years ago due to incentives offered by the government for higher MPG ratings. This relatively low viscosity oil can eke out a slightly better fuel economy. But what concerns me is that there are also claims floating around that using 5W-20 comes at a cost to long-term engine performance/life. It's this latter claim that I really want to investigate.

So my question: is the statement that 5W-20 oil provides slightly better fuel economy at the expense of long-term engine life a true one?

Last edited by Rigamarole; 06-22-2010 at 06:15 PM.. Reason: typo
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  #2  
Old 06-22-2010, 09:18 PM
lazybratsche lazybratsche is offline
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My first guess is that there wouldn't be a very significant change in fuel economy between the two types of oil. Probably resulting a change less than 1 mpg.

I actually found a decent cite that describes typical energy losses due to oil viscosity. Here's a presentation (pdf warming) from Argonne labs on the topic, though it's not a definitive, peer-reviewed publication. Page 2 states that a typical engine loses about 1.35% of its energy to viscous losses. Page 7 has a graph showing that switching viscosity grade of 20 to 30 gives a 0.5% boost in fuel economy in current engines (the 0% friction reduction curve).

(The report on whole claims that 5% gains could be theoretically achieved from friction and viscosity reduction).

So... it's a damn small change in absolute terms. Though several such small improvements would add up to significant gains, which is something automotive designers would want to do.

And that doesn't answer your question about the effect of oil viscosity on engine life...
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  #3  
Old 06-22-2010, 10:01 PM
GreasyJack GreasyJack is offline
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I suspect in some academic sense you are trading longevity for economy using 5w-20, particularly if you're talking about driving in a hot climate. The trouble is that it's really hard to quantify longevity these days because modern car engines don't wear out like in the old days-- unless they're overheated or otherwise abused they'll generally last much longer than the car itself. So maybe using 5w-20 means your engine would have only gone 700,000 miles instead of 750,000 or something, but hardly anyone keeps a car that long so who's to say?

This is also why most opinions about oil types are utterly unfounded-- it's practically impossible to actually say anything meaningful about real world performance of one type over another when even the absolute cheapest oil you can get will keep your car running indefinitely for all practical purposes.
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Old 06-22-2010, 10:29 PM
Rick Rick is online now
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Originally Posted by GreasyJack View Post
This is also why most opinions about oil types are utterly unfounded-- it's practically impossible to actually say anything meaningful about real world performance of one type over another when even the absolute cheapest oil you can get will keep your car running indefinitely for all practical purposes.
This is incorrect.
two data points for your consideration.
I had a video from Valvoline showing the difference between three oils in extreme cold (-25 or so) Three identical engines with clear plastic valve covers were put in a freezer overnight to cold soak them The first was a 10W-30 recommended by the car maker, with a (then current) additive package, the second was an older generation 10W-30 with a previous additive package, the third was 10W-40.
the engines were started from cold.
In about 30 seconds oil could be seen to be extruded like soft serve ice cream from the lube holes in the camshaft of the first engine.
About a minute later, the second engine followed suit.
30 minutes later, no oil had yet come from the oil holes in the 10W-40 engine. At this point they terminated the test.

Second point about oil quality. A 200 hour test was planned on a modern engine to measure wear between a modern (SL or SM rated oil) and an old school (SA) oil.
Two identical engines were started After 200 hours, the engine with the modern oil showed no real wear. However the engine with the old school oil blew up 162 hours into the test.

So the absolutely cheapest oil you can buy will not keep your car running forever.
Rick
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  #5  
Old 06-22-2010, 10:55 PM
GreasyJack GreasyJack is offline
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Originally Posted by Rick View Post
So the absolutely cheapest oil you can buy will not keep your car running forever.
Okay, okay, so the absolute cheapest oil you can buy that meets the manufacturer's specifications will keep your car running indefinitely. Plus, you'd really have to try to buy an SA-rated oil in this day and age! I think even the generic 30w I put in the lawnmower is SL-rated.
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  #6  
Old 06-22-2010, 11:09 PM
Rick Rick is online now
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Originally Posted by GreasyJack View Post
Okay, okay, so the absolute cheapest oil you can buy that meets the manufacturer's specifications will keep your car running indefinitely. Plus, you'd really have to try to buy an SA-rated oil in this day and age! I think even the generic 30w I put in the lawnmower is SL-rated.
Actually with this first came up at my car company, I went and looked. SA was on the shelf at the local auto parts emporium.
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  #7  
Old 06-23-2010, 12:14 AM
Mr. Slant Mr.  Slant is offline
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I think we've had this discussion more than once.
I spent a LOT of time reading on this topic a few years back.
I've since found better hobbies.
I would submit that you will have trouble noticing the difference in gas mileage between 5W20 and 5W30 on a single passenger car.
If you own a large fleet of cars, 5W20 might save you a few hundred dollars per year.
The savings are, depending on who you listen to, somewhere between 0.3 and 4.0%. [1].
It's not really worth worrying about 4% on my private vehicle, although it's nice to save $50 per year. The big environmental and energy security advantages come from improving the entire US vehicle fleet... it adds up.

Your vehicle's oil doesn't really heat all of the way up until you've been driving for a solid half an hour. Your radiator temp may max out 5 minutes after you start, but that's a leading indicator vs motor oil temp.
If your drive to work is only 20 minutes, your 5W30 isn't acting like a hot 30-weight oil... rather, it's too cold, and is acting more like a hot 40-weight oil, etc [2].

I ran across a study by a Japanese university that showed marginally higher wear with 5W20 vs 5W30 during bench tests on a smallish Japanese 4-cylinder. The 'margin' in question was freaking tiny. I'll try to dig it up, but no promises. I seem to recall that I found it as a link from a page a Shell engineer had up a few years back.
Most modern cars have engines with reliability such that you'll go through three transmissions and a pair of water pumps before any of the internally-lubricated hard parts even think about wearing out. If you're in any state bordering the Great Lakes, add an entire set of body panels to that last sentence.
That being said, on a technicality, Rigamarole, your answer is 'yes'.
The reason it's a technicality is that both the increases in fuel economy and wear are quite small.

If I had your car, I'd just use whatever name-brand API SM 5W20 was cheapest at my local retailer or garage. Your engine will likely outlast the rest of the vehicle, and if your drives to work are on the short side, it'll probably make your car last longer as well.

[1] This PDF cite suggests 2-4% as the likely difference changing motor oils can make in your MPG. Note, however, that the data isn't showing 5W20 vs 5W30 numbers. http://www.iea.org/work/2005/EnerEffTyre/calwell2.pdf
[2] For a fascinating treatment of motor oil from the perspective of a heart surgeon, read the first 10 links on this page: http://www.bobistheoilguy.com/aehaas/
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  #8  
Old 06-23-2010, 12:42 AM
Terry Kennedy Terry Kennedy is offline
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Quote:
Originally Posted by Rigamarole View Post
I have a fairly new (model year 2007) Mazda and the manual recommends 5W-20 oil, which is what I have been using to this point. But I did some Google searches to educate myself about oil and there seems to be a point of contention over whether this is the "best" oil to use. I'm finding a lot of conflicting and unsubstantiated opinions on this so I wanted to turn it to you guys.
Rule 1: If the car is under warranty, do what the manufacturer says.
Rule 1A: If you think the manufacturer change interval is too long, change at whatever frequency you like with the manufacturer-recommended oil while under warranty.

Rule 2: If you aren't planning on keeping the car "forever" (like past 150K miles), don't worry about what oil is "best". Use the oil the manufacturer recommends or one with the same viscosity and service rating.

Rule 3: If you have some sort of special engine (for example, the one in my race car) hire an independent oil analysis consultant and have him examine every batch of oil from the first time the engine was cranked to monitor any trends or problem areas. I use Terry Dyson.

Note that people have spent $50K and up, just on the engine. And for those engines and transmissions, even things that are normally "lifetime fill", like the gearbox oil, get changed every few thousand miles or after a race. These engines also have things like pre-lubricators that drop a quart or two of oil before the engine is even cranked.

Quote:
Originally Posted by Mr. Slant View Post
Your radiator temp may max out 5 minutes after you start, but that's a leading indicator vs motor oil temp.
I can't say for certain on other cars, but on the race car I have degree-accurate thermocouples in the water and oil in the block, as well as some other key places. My oil temperature goes up a lot faster than the water temp does - from a dead cold (room-temperature) start, by the time the water has hit 160 the oil is already at 180. Eventually they'll equalize in the high 180's, at which point the radiator fan will come on and they'll both drop roughly equally back to the 160's.
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  #9  
Old 06-23-2010, 12:44 AM
Rigamarole Rigamarole is offline
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So Rick, should I keep using 5W-20 as my manual recommends if I care about my car's longevity?
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