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Old 05-24-2009, 07:35 AM   #10 (permalink)
stevey_frac
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Join Date: Jun 2008
Location: Cambridge, ON
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Jalilah - '07 Chevrolet Cobalt LT
90 day: 40.57 mpg (US)
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Quote:
Originally Posted by evolutionmovement View Post
I wouldn't bother with 0W. Stuff pours like water it's so thin and will find places to leak from. Also, the 0W doesn't protect as well at high temps as 5W/30 and is, IMO, unnecessarily overkill low for cold temps. If you were going to change at all, you'd be better off going to 10W/40 for the summer than 0/20, but the 5/30 is probably the best all around choice. What does the owner's manual recommend?
Ow30 is exactly the same viscosity when warm as 10w30. Furthermore, the fact that it's lighter when cold IMPROVES wear. Engines are designed around a certain operating temperature and oil viscosity. When the engine is cold 10W30 is about 5-6 times too thick. The oil pump has a much harder time pumping this very thick oil around and flow suffers. Your oil PRESSURE is probably fine. But oil flow is severely limited.

Real values of real oil. (Amsoil used because they publish it all if you know where to look, not because i'm a fanboy)

AMSOIL 0W30 SSO

Kinematic Viscosity @ 100C, cSt (ASTM D445) = 10.3
Kinematic Viscosity @ 40C, cSt (ASTM D445) = 56.6


AMSOIL 5W-30 ASL

Kinematic Viscosity @ 100C, cSt (ASTM D-445) = 10.8
Kinematic Viscosity @ 40C, cSt (ASTM D-445) = 60.7

AMSOIL 10W-30 ATM

Kinematic Viscosity @ 100C, cSt (ASTM D-445) = 10.7
Kinematic Viscosity @ 40C, cSt (ASTM D-445) = 62.0


(note, the cold numbers only get worse as it gets colder)

People mistakenly believe that the problem with oil is that it thins as it heats up. This is a really the wrong way to look at it. The problem with oil is that it thickens as it cools! Most engines are designed around a 100C or 212F operating temperature. Your oil will stay at around this temperature unless your drag racing it once it has warmed. Until the oil heats up though, your pumping this very viscous fluid around, and it's probably 6 times too thick. It's have a hard time reaching all the parts of your engine in sufficient quantities to be useful. You wouldn't reccomend someone run an SAE 150 oil would you? But this is more or less what your doing every time you start your car from cold. It's also one of the reasons why most wear occours during cold starts.

This is why most engines now use 5W30 instead of 10W30. The only reason these engines don't get back-spec'd to a 0W30 oil is because it can't be done properly with a conventional oil at this point, so 0W30 oils are synthetic and expensive. Run a 0W30 oil. Your engine will like it. Your fuel economy will like it.

Complaints that it 'pours like water' and that 'it'll find new places to leak' are unfounded, and unreasonable. Even 0W30 is 5 times to thick at 40C for the engine. It's just closer to 'normal' then 10w30. They are ALL the same thickness at operating temperature, and all provide similar levels of protection.

I would caution you against putting a xw40 oil in your car. Unless your driving like a bat out of hell, or pulling a trailer through death valley, or your commute involves climbing pikes peak at 70 MPH, it's going to be too thick even when warm for your car. Not by much, but still too thick. Roughly 50% thicker then design spec for most cars.

AMSOIL XL 10W-40

Kinematic Viscosity @ 100C, cSt (ASTM D-445) = 15.7
Kinematic Viscosity @ 40C, cSt (ASTM D-445) = 104.4


Now... when your cold starting that 10w40. It's going to be more then 10 times too thick! You would end up accelerating wear (significantly, but not catastrophically) and ruining your fuel economy. The only logical argument i've heard for running these thick oils is if your main bearings are worn badly enough that they literally hemorrhage oil, and the thicker oil prevents this somewhat, and allows your car to maintain oil pressure despite this. This prevents oil starvation of the top end, and is desirable.

EDIT: Re 0w20

It's a touch thin, but not terribly so.

AMSOIL 0W-20 (ASM)

Viscosity @ 100C, cSt (ASTM D-445) = 9.0

If your hypermiling i'd be inclined to think that you'd be ok. That being said, I wouldn't do it, mostly because i'm conservative about that kind of thing.
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Last edited by stevey_frac; 05-24-2009 at 07:42 AM..
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