I just did a huge write up on oil weight (under I. Maintenence, C: Routine Tasks, 1. Oil changes) and I am definately not finished on the subject of oil in general. However, I'm feeling a bit worn out of typing about oil now.

Here is what I have. Again, feel free to edit or remove any inaccuracies as needed.

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Oil Weight

Before going into discussion about what weight of oil you should use in your engine, it would be helpful to understand exactly what the different weights mean, and what is used to measure the weight. We should also look at how multi-viscosity oils work.
If you already know all of this information, then just skip to the next part.

So, let's talk oil weight. What exactly does the weight rating of oil mean? In the most simple terms, the weight of the oil refers to it's viscosity, or thickness. A low weight oil is thin and flows very easily. As the weight of the oil increases, it becomes progressively thicker and more resistant to flow. Obviously, a 0 weight oil is very thin and is able to flow very easily compared to a 40 weight oil, which is quite thick and flows with more difficulty.

Considering all this, the next question that you would ask might be "What is used to measure the various weights?" Good question. According to the Automotive and Industrial Lubricants Glossary of Terms,
The paragraph is pretty much self-explanatory, but in case you don't understand, it basically says that at a given temperature, oil is run through an orfice and the faster it runs through the orfice, the less viscous (thick) it is and the less weight it is rated as. For example, a 0 weight oil would run through the orfice much faster than a 50 weight oil at a given temperature. Of course, there are other ways in which oil is rated, but this is a well-known way to measure oil viscosity.

Now that we know how what the weight of the oil means and how it is rated, we should talk about multi-viscosity oils. What exactly are they and why were they created? Multi-viscosity oils (such as 10W-30) have two different viscosities at different temperatures. The first number is the weight of the oil when it is cold. The second number is the weight of the oil when it warms to a certain temperature.

The "W" in multi-viscosity oils means that the oil has met the specified viscosity at 0°F and is suitable for winter use. Many confuse the "W" for weight, which it is not.

Anyway, the way a multi-viscosity oil works has to do with how polymers interact with the base oil. At very low temperatures, polymers in the oil are "coiled up" and allow the oil to flow at the "W" rating. As the oil begins to heat up, the polymers in the oil "unwind" into chains that prevent the oil from thinning as it would if the polymers were not added. In effect, the oil only thins to the rating of the second number. For example, a 10W-30 oil would flow as a 10 weight oil when cold and then flow as a 30 weight oil as it warms to operating temperature. A good way of looking at this would be to think of a 10W-30 oil as a 10 weight oil that will not thin more than a 30 weight oil would when it is hot.

So now we know how oil's weight is rated, what is used to measure the oil to obtain the rating, and know how multi-viscosity oils work. Now, what weight oil should you use in your engine, you ask? Let's take a look.

Obviously, for the best lubrication of the engine, a multi-viscosity oil will work better than a single weight oil. That said, which multi-viscosity oil that will work best for your car varies depending on conditions such as location, temperature and type of engine. For example, when choosing a multi-viscosity oil, a person who lives in a place that almost never gets temperatures above 30°F will need a different weight than a person who lives in a location where the temperature is regularly 80°F or higher. Since we are dealing with the 60° V6 (2.8L/3.1L), I won't go into the debate of engine part clearances and whatnot since all of them should be pretty similar to one another. If you're one of the people who knows they need a different weight due to engine modifications and the like, you probably already have a pretty good idea of how engine oils work and what kind of weight you need to use.

Anyway, when picking a multi-viscosity oil, you should choose one carefully. Try to use the oil with the smallest span of viscosity correlating to the temperatures you will encounter. In other words, choose an oil with the least amount of difference between the two numbers so long as it goes along with the temperatures you will get in your location. For example, if you regularly encounter below freezing temperatures in your location, use an oil designed to work with those temperatues such as 5W-30. However, if you don't, choose the 10W-30 instead.

What is the reasoning behind this, you ask? Well, in order to obtain such a great distance between cold and hot viscosities, oils have to have more polymers added into them. Oils are what lubricate the engine, not polymers. Thus, the more polymers added into the oil, the less actual oil will be there to lubricate the engine (which is obviously a bad thing). Additionally,
polymers are very prone to shearing and thermal breakdown which can cause sludge and other problems.

While it is up to you which oil you choose to put in your car (again, depending on your conditions), there are a few oils which are VERY RARELY recommended because of what has been stated already. Most manufacturers recommend against using weights such as 10W-40, and some car manufacturers may actually void warranties on cars if it is used. 20W-50 has the same spread as 10W-40, but since it starts with a heavier base to begin with, it
requires less polymers to do the job. It is also usually unnecessarily heavy and will require more work for most engines to flow which means less power to the wheels, lower gas milages, ect.

 

Nice. Very nice.

 

This looks good.