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alright, well I gathered up the manufacturer specs on a good number of synthetic oils ... so here's my question:

how do you determine which oil is the "thinnest" or "thickest" ? ... my guess would be the kinematic viscosity @ 100c ... but I'm not sure. Or would it be the viscosity index?

I've been using Amsoil 0w30, but upon inspection of the spec sheet data, both the kinematic viscosity and the viscosity index (I'm guessing they're related) are both HIGHER than the 5w30 oil. Wouldn't this mean that the 0w30 is actually a thicker oil than the 5w30??

so ... right now I'm trying to decide whether I should use Mobil 1 0w30, Amsoil 0w30 or Amsoil 5w30 ...

 
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Well, I’m not really a guru of oil, but there is a big question to that viscosity you are asking about. What temperature where the reading taken at?

The viscosity is simply the ability of the fluid to flow or it's resistance to flow, depending on what book you read. The lower the number, the "thinner" the fluid or the more easily it will flow.

With that in mind, I go back to the temperature question. If we look the rating of oil, you see there are 2 numbers given, 0w30, 5w30 ect. The first number is the viscosity at 30C I think and the second number is the viscosity at "operating temperature around 100C if I remember right. Anyway, the first one is the cold temp and the second number is the hot temp. So if we compare 0w30 and 5w30, it would make since that 0w30 is "thinner" or less viscous at low temperature than 5w30, but we would expect that at operating temperature they will be close to the same viscosity.

Make sense?

On a different note, there is something called sheer stress that oil is subjected to. Think of this as when you are really close to someone (physically) and you try to move past them, you may bump them right? Well, if you are going fast enough and hit them hard enough, you break off an arm or something right? Yeah, sorry for the gory example, but it work well. The same thing happens to oil, if the oil get flowing fast enough and under the right conditions it literally sheers apart.

Now what does this have to do with anything? Well, the larger the difference between the cold and hot viscosities, the more prone to sheering the oil is. Why is this? Well, I'm not 100% sure, but I've seen enough UOA's to know this is true So what I am saying is that you should make sure those two viscosities are close together to get any sort of good life out of the oil. Now if you change the oil every 2,000 miles and drive with a light foot, it doesn't matter. But this is one reason why 5w20 is becoming more common for extended oil changes.

 
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I have found that most answers regading oils can be found here: http://www.bobistheoilguy.com/

 
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You beat me to it!
This site is like college class for oil heads

 
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I've read quite a bit on that site.

it doesn't answer my question.

jr: I know what viscosity is and I know what oil weights are. my question was specifically from analyzing the different kinematic viscosity readings, if you could determine which oil would be thinner.

 
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well... I'm not exaclty sure what you are looking for, so let me through some terms out for you ponder:

Kinematic viscosity (KV) is absolute viscosity divided by density. So this means that it takes into account the change of density that a fluid has when it is heated. This of course means you have to know the density to find the true viscosity.

Now of course a lower KV could mean one of two things, the density is higher or the true viscosity is lower.

Now the real problem I have is that you want to equate KV ot being "thinner" but the question is what is thinner? By thinner do you mean less dense? or maybe less viscous? For an oil, we generally want more dense, less viscous, more dense will allow more molecules of oil per unit volume. This is good for heat adsorption and various other little things. But generally more dense means high viscosity, which can be translated to thicker by most people. This is where many additives come in to change the properties.
Hope that helps a little

 
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the site might not already have the answer, but youd be better off asking on that forum. id trust their responses more.

 
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that's what I meant