The Difference Between Synthetic And Normal Motor Oil
What The Numbers On A Bottle Of Oil Mean Mean?
I’ve had many customers who have had their oil changed at my shop. Many of them ask about synthetic oil and wonder what the
difference is. In fact, a lot of them have been warned not to replace their oil with synthetic. They’ve been told that it’s a
really bad idea, but it’s not! Most of those making the warnings just simply don’t know what they are talking about.
A recent customers reaction to this news made me think that maybe it would be a good idea to share with my readers what oil is all
about, and what the differences are between regular oil and synthetic oil.
Here’s the basics. Car engines need something to lubricate all of the moving parts in the engine. That’s what oil is for. If there
were no oil in your engine, the parts would heat up due to friction and essentialy melt together. That would be bad, very bad. So,
we put oil in the motor to prevent this. The oil makes everything slick and lets the parts move without causing friction.
That’s why it’s important to keep your oil level full and change it when required, so it doesn’t become worn out and worthless.
Oil keeps everything moving the way it should.
The primary thing about oil is it’s viscosity. Viscosity is the level of thickness which determines the slowness of flowing. For
example, thick honey has a high viscosity, and water has a low viscosity. Understand?
In an engine, we essentially want oil to be sort of dual purpose in terms of viscosity. It has to be able to flow easily when we
first start up the car, so the engine is not running without lubrication, but it needs to remain thick and protective enough when
the engine warms up and is up and running at full force. The problem is that oil doesn’t really work like that. It needs some help
to do that.
So, What do those numbers on the bottle 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
So, the first number is how viscous the oil is when it’s cold—as in o°F cold— 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
parts 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 that all about?
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 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
air-cooled engines, 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.
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.
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
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. In fact, most oils are packaged
by weight at the same facilities. The only difference is the label on the bottle. There are a very few exceptions, like Valvoline
for instance. That’s not a plug, just a fact. That brings up another fact that everyone should be made aware of. The difference in
cost with “economy” oils is that some of them are actually re-refined. That’s a process where used oil is cleaned up, repackaged
and resold cheap. I wouldn’t recommend it, even on a tight budget.
So there you go. Everything you always wanted to know about oil, and perhaps a little bit more!
Drive safe and thank you for reading!