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Old 08-12-2010, 06:41 AM   #1
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Oil viscosity - more difference than I would have thought

So my wife has been trying to maximize her MPG in her driving. She has a '98 Park Avenue (non-SC) with the 3.8.

She pays a lot of attention to the MPG readout. She's been proud to work it up to 27.6 MPG - not bad for such a big car.

At the last oil change, I decided to use up some old 10W 40 oil instead of the usual 10W30. Almost immediately afterwards, her MPG started dropping, and settled out around 26.6MPG (even with her attempts to improve her driving). She asked me what was wrong.

As soon as I realized that the change started to occur around the time I changed her oil, I did some quick calculations and realized that it would be worthwhile (even with the cost of synthetic) to re-change her oil back to regular viscosity. I drained her oil and filled it with 5W30.

As soon as I changed her oil, the MPG indicator started rising again almost immediately.

This past week we drove out to Michigan (about 2400 miles round trip) which was all highway, so the MPG is sitting at 30.5 right now, but we'll see where it settles out when she resumes "normal" driving.

But a 1 MPG difference is MUCH more than I thought it would be, based only on the oil viscosity! I wonder if 5W20 would get her another MPG...

-Bob C.
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Old 08-12-2010, 07:51 AM   #2
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Re: Oil viscosity - more difference than I would have thought

Odd, I was just about to post a similar thread with the opposite results.

About a month ago (I think), I removed the small upper grille block that raised my IAT. I also changed my commute route. My MPG went up even during the week I took off where I didn't commute.

During that week off I changed from Castrol European Formula 0W30 synthetic to Castrol 5W40 synthetic and my MPG seems to have jumped again.
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Old 08-12-2010, 08:49 AM   #3
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Re: Oil viscosity - more difference than I would have thought

It's not odd at all. Your test sample is so small it is an unreliable indicator of anything.
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Old 08-12-2010, 09:03 AM   #4
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Re: Oil viscosity - more difference than I would have thought

The numbers on the label just refer to a standardized range of viscosities. The true synthetic 0w30 Castrol is thick for a 30 weight. The 5w40 is still thicker, but oils thicken with use, and the old 0w30 may have been thicker than the new 5w40.

http://www.bobistheoilguy.com/visc.html
http://www.castrol.com/liveassets/bp...syntec_usa.pdf
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Old 08-12-2010, 09:25 AM   #5
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Re: Oil viscosity - more difference than I would have thought

I agree about the small sample size, but in the past 40,000 miles with this car and scangauge I've gotten pretty good at seeing past that.

Actually, thinking about the oil, I think the last time it was changed was the time I had a coupon for the dealer to do it cheaper than I could buy the oil. They probably put in 5W40 too.

Before this summer is over I may hit some new record tanks.
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Old 08-12-2010, 11:48 AM   #6
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Re: Oil viscosity - more difference than I would have thought

Hi Bob,
I have the same experience as you: I changed from an unknown viscosity (bought second hand) to 0W20 and my mpg rose immediately!
Still didn't empty a full tank with that oil, but I will certainly set a new 'personal best'. To give you an idea: consumption on dashreading went from 5,5 (previous tank) to 4,6 l/100km for the current tank. It's not all because of the oil (holiday=less traffic and higher temps).
I'm talking about my Honda Civic i-DSI. Longer time will tell what the real gains of the oil are... but it looks good.
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Old 08-12-2010, 04:49 PM   #7
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Re: Oil viscosity - more difference than I would have thought

i could go along w/ Bob on this one. it's possible some manufacturers use 0w20 to achieve better EPA estimates and fewer emissions, but at the expense of lubrication.
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Old 08-12-2010, 09:28 PM   #8
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Re: Oil viscosity - more difference than I would have thought

Quote:
Originally Posted by bowtieguy View Post
i could go along w/ Bob on this one. it's possible some manufacturers use 0w20 to achieve better EPA estimates and fewer emissions, but at the expense of lubrication.
It's also possible that they're able to get away with the thinner oils because the cars burn so cleanly and when you burn cleanly, hopefully it's complete combustion. Rich combustion is usually what pollutes oils and when you've got good thermal management, and computer control for knock, this allows for extended drain intervals especially if you do "easy" driving. Main problem with "thinner" oils is that they usually burn off when they get "older" but since the 0W-20 oils are all synthetic now (not true PAO synthetic but good enough), they're not as problematic as they would be many moons ago.

It pretty much comes down to this:
Cars run cleaner and therefore pollute the oil they use much less than they once did.
The Oils have gotten more advanced, to the point of where they maintain their useful life much longer than they once did.
Engine tolerances have gotten tighter so the use of heavier oil becomes a detriment to engine longevity since looser tolerances equals more wear.
Therefore, the need for a higher viscosity oil is reduced since usually they use higher viscosity oil to make up for the fact that the oil needs to absorb pollutants and particulate matter.
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Old 08-13-2010, 06:58 AM   #9
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Re: Oil viscosity - more difference than I would have thought

Don't forget one of the most important things to let an engine run on 0W20 is an oil pump with a high flow on low revs (in combination with the narrower tollerances).
Bowtie, I don't really understand what you mean with 'it's possible some manufacturers use 0w20 to achieve better EPA estimates and fewer emissions, but at the expense of lubrication.'. I assume you talk here about the fact that a thinner oil does not perform as good as a thick oil at very high temps in combination with high pressure? But these circumstances do not occure that often. What does occure at every cold start is that a thick oil is by far not as fast where it needs to be than the thin oil. So lubrication might be better in real life with the thin oil.
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Old 08-13-2010, 07:30 AM   #10
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Re: Oil viscosity - more difference than I would have thought

a true synthetic 0w30 is just as good at start-up as 0w20, but's it's FAR superior in lubrication, longevity, heat protection, cold weahter performance,etc. what i'm suggesting is that manufacturers use the cheapest methods available to achieve certain #s, irrespective of the interest or longevity of the vehicle.

remember toyota's recent issues? to be fair, shortcuts(and cost cutting) has affected several auto makers in recent years.

edit: 0w20 is likely fine for winter/cooler climates.
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