Here's The Difference Between Synthetic And Normal Motor Oil, And What The Numbers On The Bottle Mean

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I just changed the oil in my Nissan Pao last week, and in doing so I was warned—several times—that my plan to replace the conventional oil in my car with synthetic was a terrible idea. But you know what? It’s not! That got me thinking that maybe a little oil refresher might be of use. Not to you, of course, you know all this stuff. For everyone else.

Let’s start with the basics: car engines use oil to lubricate all of the many moving parts. Without oil, parts would rub together, causing damage and friction, which in turn would cause heat to build up, which would eventually get so hot that parts would expand and everything would seize up into a miserable heap of slag. Nobody wants that. So we liberally coat everything in golden, slickery oil to make sure the worst doesn’t happen.


The key criteria we use to grade oil is viscosity. Viscosity is, basically, the level of gooey-ness of oil; the thickness, the slowness of flowing. Molasses has a high viscosity, for example, and water has a low viscosity. Got it?

In an engine, we essentially want oil to be sort of dual-natured, viscosity-wise. We want it to be able to flow readily and easily when we first start up the car, so we’re not running an engine without any lubrication, but we want it to remain thick and protective enough when the engine is up and running. The problem is that oil doesn’t really work like that—at least not without some help.


What do those numbers mean?

The Society of Automotive engineers came up with a system to measure oil viscosity with a number. They perform a test where they time the oil as it makes its way through an orifice of some kind. The longer it takes the oil to get through, the higher the number, and the more viscous (thick, goopy) the oil is. So, higher numbers means higher viscosity.


Sometimes this number is referred to as the oil’s weight. Basic oils can have just one number—say, an SAE 30 oil. These oils will have a greater viscosity when cold, and thin out—often significantly—when hot. That means it’ll be hard to pump when you start a cold engine and could be too thin when running in a hot engine.

To get around this, most cars use formulated motor oils with two numbers, like 5W-20 or 10W-40 or whatever. Here’s what those numbers mean:


So, the first number is how viscous the oil is when it’s cold—as in o°F cold (or, depending on the grade, much colder, as mentioned in the comments!)— which is how well it’ll flow when you first start up the car on a cold morning, so you want a lower number here, so the oil isn’t too thick to get pumped around to all the vulnerable bits in the engine. To let you know this is the cold number, it’s followed by a “W,” for “winter,” which is, duh, cold.

The next number is the viscosity of the oil when the engine is hot and running, which you’d want higher than the cold number.


The way these multi-viscosity oils work is by starting with a “base oil” and then using additives to adjust the viscosity of the oil at a secondary range of temperatures. Engineering Explained talked to an oil engineer (who, admittedly, works for a specific company so she shills a bit) and she gives a good explanation of all this:

What about synthetic vs. conventional oil? What about that?

This video also brings up synthetic oils. Synthetic oils differ from conventional oils in that where conventional oil is just refined from crude oil, synthetic oils are refined from crude oil, then distilled down into the component molecules. This helps separate impurities from the oil, and allows for the manufacturer to “customize” the oil’s fundamental molecules.


Doing all this molecular juggling is known as the Fischer-Tropsch process, and is way beyond my meager brain-CPU’s ability to process. But, it works, and it makes motor oil with much more flexible properties and better mechanical properties at extremes of temperatures.

As far as what oil you should use for your car, the best bet is to research and find out what the manufacturer originally wanted you to use in that engine. On old, low-compression, high-mileage engines, I tend to like to use thicker oils, and, in the case of my air-cooled Beetle, where the oil does a lot of cooling work as well, I’ve used 20W-50 oil (which is about as thick as you can usually find) for high-mileage engines. Most cars likely won’t need such thick oil.


Also, a general rule of thumb is that in the winter, you’ll want a thinner (lower number) oil than in the summer, but the safest bet is to research your own engine.

This other Engineering Explained video has some great demonstrations of the differences in synthetic and conventional oils:

The big takeaway is synthetic oil does its job better even after it’s been in use a while compared to conventional oil. It’s also able to be formulated to viscosity ratings conventional oil can’t, like 0W-15 or something that can still flow well even at extremely cold temperatures.


There’s a lot of myths about synthetic oil, like what I referred to at the beginning of this article, most notably that you can’t switch between synthetic and regular oils as you please. Usually, it’s said that if you started with synthetic oil and then switch to conventional, you can’t go back.

That’s bullshit.

You can switch between oil types as often as you want. Hell, you can even buy oils that are mixes of synthetic and conventional oil! It’s just not something to worry about.


That said, there are some engines—usually modern, high-tech, expensive engines—that have tolerances and requirements that necessitate only using synthetic oil. The issue here isn’t the oil, it’s the engine, but even with engines like that, if it’s an emergency and you have no other oil around, putting in conventional oil is always going to be far better than running with too little oil.

Synthetic oils are overall better, so why don’t we just use that for everything? The answer is, like for pretty much everything, price. Not all engines really need synthetic oil, and it’s more expensive, so why waste money?


If money was no object, sure, use synthetic, but if you’re running a relatively low-tech engine, you can save your money, usually.

Are more expensive motor oils with lots of additives and vitamins or whatever worth it?

Uh, no. Not really. We looked into this, and it doesn’t seem like it makes that much difference.


Will drinking motor oil kill you?

No, no it won’t. But it’s not great for you, so, you know, try not to use it as a mixer.