If you’ve ever had to go buy motor oil before you may have noticed that there are many different kinds which are labelled with a series of numbers and letters on the bottle. For new drivers this can be a confusing situation because the label typically doesn't explain what those numbers mean. This is the kind of thing that you need to know about it before you go pick up the oil. And, worse, you need to make sure you're getting the right kind of oil for your vehicle. There are synthetic oils and other formulations that don’t work in every automobile. So, you need to know what kind of oil your car requires, and it's good to know why you're getting that kind of oil as well. So let's take a look at what the numbers on a bottle of motor oil need and how you can read them so that they'll make sense in the future.
What Does the First Number in Oil Mean?
A typical bottle of motor oil has two numbers on it separated by the letter W. The number before the W is a reference to the viscosity of the oil in cold weather conditions. The W that you're seeing in the motor oil number stands for winter. So, the number preceding the W is the viscosity of the oil in winter. The lower the number the more viscous, or thick, it's going to be in cool weather. You want oil that is going to be less viscous in cold weather for most vehicles, especially if you drive in a cold-weather climate at any point in time. If the oil thickens up too much in cold weather, it's going to have a difficult time circulating through your vehicle.
As an aside, you may also notice on some bottles of motor oil that before that first number there are the letters SAE. SAE stands for the Society of Automotive Engineers. They are the ones who established the numerical code that grades motor oils according to the characteristics of their viscosity. This had to be done to make it easier for drivers to understand what they were putting in their cars because oil viscosity changes as temperature changes. Multi-grade oils exist because they are able to provide protection from low temperatures through the high temperatures.
So, just to clarify again, the lower the number before the W in a motor oil, the better it's going to work in cold weather.
What Does the Second Number in Oil Mean?
The second number in the motor oil number references the viscosity of the oil during normal operating circumstances. This is how it works when it gets up to temperature, usually around 100 degrees Celsius. The most common kinds of oil are 5w30 or 10w40. The 5w30 is a less viscous oil, so it's likely going to work a little better at cool temperatures, but it will be thinner at high temperatures. Both of these kinds of oil are used frequently in most vehicles because they operate pretty well in the standard range of temperatures that most of us drive in.
There are circumstances when these two weights of oil might not be ideal, but that represents temperature extremes. If you live in the northern part of Alaska, you might want a 0w type of motor oil. Conversely, if you're living in Death Valley or in the driest, hottest part of the Australian desert then you might want to go all the way up to a 20W40 motor oil because it's not going to ever be experiencing those super low temperatures.
What Does it Mean if Oil Only Has One Number?
There are two kinds of motor oil that you may find on the shelves. Multi-grade motor oil has those two numbers, like 5w30. But you may also find motor oil listed as something like SAE 30. It's good to know what the difference is between these two kinds of oil.
Standard Oil: If the oil you're looking at has just that one number, like SAE 30, that means that it is a 30 weight oil and that is the viscosity of the oil will have all the time, it's not meant to handle temperature extremes. Something like an SAE 30 oil would be used in heavy-duty equipment, like construction or farm equipment for instance.
Multi-Grade Oil: The motor oils that have two numbers, like a 5w30, don't mean that the viscosity is gained or lost as the temperature changes so much as it means that when the oil is cold it will function the way a 5 weight motor oil would and when it is warm it will function the way a 30 weight motor oil would. So actually, what's happening is the viscosity decreases as the oil warms up.
How Do I Know Which Oil is Right for My Car?
The easiest and best way to find out what kind of motor oil your car should be using is to simply look at your owner's manual. Everything you need to know about what's best for your vehicle is actually going to be in the owner's manual. However, we don't always think to look there in a pinch.
Your owner's manual is going to tell you precisely what weight of motor oil is going to work best in your vehicle, although the brand really shouldn't matter that much and it's not something you should worry about. A mechanic might recommend specific brands to you, but the formulation should be almost identical from one brand to another for the same weight so it's generally not relevant to worry about that. It's also possible that your owner's manual can recommend an alternative weight of oil that you can use in your car should you not be able to find the exact recommended kinds.
Can I Use 5w30 Instead of 10w40?
A 5w30 motor oil is a synthetic oil that has a viscosity of 5 in low or winter temperatures and a viscosity of 30 at higher temperatures, usually around 100 degrees Celsius. That's the boiling point for water. On the other hand, a 10w40 motor oil has viscosity of 10 in those low temperatures and the viscosity is at 40 at higher temperatures. So, what exactly does that mean in terms of how these oils operate in a vehicle?
A 5 w30 motor oil is less viscous than the 10w40. That means it's thinner at low temperatures and at high temperatures. So, in cold weather if you're worried about how your oil is flowing, the 5w30 is going to flow more smoothly at low temperatures than the 10w40. When you get up to high temperatures, the 10w40 is going to be thicker.
It's possible to use a 5w30 instead of a 10w40 in a car if you want to and in general most cars tend to work better with a 5w30 then a 10w 40 anyway. There are occasions when a 10w40 would be better or for oil for a particular vehicle, but you really should be checking your owner's manual to determine what you should be running in your vehicle and following those recommendations.
If you check your owner's manual there's a good chance it will recommend 5w30 and also list 10w40 as a reasonable alternative if you can't find the 5w30. You really should double-check to make sure however, because that may not be exactly the case with every single vehicle and we certainly can't claim to know what you're driving. When in doubt, always refer to your owner's manual.
What Happens If I Put the Wrong Oil in My Car?
As we said, the brand of motor oil will have almost no effect on your vehicle whatsoever. So, don't worry if you didn't get the exact right branding. However, when it comes to viscosity you may have an issue if you pick something that is wrong for your vehicle. If you use the wrong weight of motor oil it's going to affect your car's ability to properly lubricate the engine and other parts. That in turn can end up shortening your engine life drastically. If your engine requires a 5w30 and you put in a SAE 50 for instance that could be a definite problem. Likewise, if you're supposed to use synthetic oil and you don't, that could cause some major issues as well. It really depends on the make and model of your vehicle however.
If you have added the wrong oil, there are some symptoms you can be on the lookout for.
Hard to Start in the Cold
If you've chosen an oil with the wrong viscosity at cool temperatures, then on a frosty winter day you may find your vehicle giving you a lot of trouble when you try to get it started in the morning. If your oil is too thick because the temperature is too low, your engine is going to be extremely sluggish.
If you use a motor oil that doesn't have the right viscosity for high temperatures, the oil can start breaking down as you subject it to those higher temperatures. If it stays hot enough for long enough it won't be able to properly lubricate your engine and can cause friction and burning inside as well.
Bad Fuel Economy
If your motor oil is not the right viscosity, then your engine is going to be struggling to work the way it's supposed to. That in turn is going to cause your fuel economy to tank on you. Since the thicker oil is increasing the resistance of the moving parts in your engine, it's going to struggle to do its job more and you're going to end up burning through more fuel without getting any kind of benefit for it. That means you're going to be spending more at the gas pumps as a result. If this is the case, you're going to need to switch to a thinner kind of motor oil to help improve the situation.
This is an uncommon symptom but not completely unheard of. If you start using synthetic oil in an older vehicle, it may start leaking because it wasn't designed for that kind of oil. Synthetic oil has an ability to leak through tighter areas than standard oil does. Older cars were designed to use conventional oil and not synthetic oil, the switching from one to the other could cause problems for you. It won't necessarily damage your vehicle or the way it performs, but since oil can potentially leak out your going to end up having to add oil more frequently as a result, and of course you're going to have to contend with oil leaks in the driveway as well and potentially some burning smells if it contaminates your engine bay. The best thing you can do if this is the situation, you've used synthetic oil when you're not supposed to, is to switch back to standard oil when you get a chance.
Another unusual side effect of choosing the wrong kind of oil is a ticking noise you'll notice in the cold weather. This happens if your motor oil is too thin to properly work in the cold weather. Just after you start up the vehicle, you'll hear this sound and it should fade away as the engine warms up. This is because, if your motor oil is too thin, it's not properly coating all the moving components in your engine so what you're hearing are the metal components clicking together as they move around without the proper lubrication. Likely the sound is caused by the valve lifters hitting against other parts in your engine. If you swap for an oil with better viscosity in the sound goes away, you know that was the problem.
The Bottom Line
Knowing what kind of motor oil your engine uses is a key part of making sure you are able to maintain your vehicle properly. It's going to save you money in the long run if you know what kind of oil you're using and why you have to use it. Think of it like understanding why you need regular gasoline versus diesel, or why you'd want to use green coolant instead of orange coolant. It's just one more way you can understand how your vehicle works and what's best to keep it working in a way that saves you money.