The Ultimate Guide to Motor Oil

Motor oil is the most important fluid in all vehicles with an internal combustion engine.

And oil maintenance is hands-down the most important vehicle maintenance activity you can and should perform. Regularly.

Through this guide, you will learn all the information you need to tackle this essential task with ease and confidence and keep your vehicle running at peak performance for years to come.

What is Motor Oil Anyway?

Motor oil is a lubricant generally derived from crude oil. Yep, motor oil derives from the same good ol’ fossil fuel that we all know.

An oil extraction site at sunset

And it is an essential requirement for all internal combustion engines (ICEs).

Why does My Vehicle Need Motor Oil?

All internal combustion engines rely on oil for operation. Oil serves three main functions:

  • lubrification
  • cooling
  • cleaning

Oil’s main job is to lubricate components: it creates a thin cushioning film between parts that prevents them from touching and rubbing together as the engine runs.

It reduces friction, and as it flows it cools the involved components.

And as it circulates, it cleans the engine, carrying away dust, dirt, and combustion residues to the oil filter to be captured.

Without oil, an engine would not work: it would be severely damaged by the friction and overheating, and break down in a matter of minutes.

Oil is essential in practically all vehicles. The only vehicles that don’t need oil are those that have fully electric motors, as they don’t have internally moving components that need to be lubricated.

As any good mechanic will tell you, motor oil is actually more important than fuel from an engine standpoint. If you run out of fuel, you just put some of it in the tank and you are good to go again. But if you run out of motor oil, your engine is ruined and you’re not going anywhere anytime soon.

How does the Oil Flow through the Engine?

A complex and intricate system of galleries and sprayers permeates all of the engine. Oil is pumped through this system and is thus able to reach and lubricate all of the moving parts.

The oil pump forces the motor oil through the passages in the engine to properly distribute oil to different engine components.

In a usual lubrication system, oil is drawn out of the oil sump through a wire mesh strainer that removes some of the larger pieces of debris from the oil.

The flow generated by the oil pump then allows the oil to be distributed around the engine. In most systems, oil flows through a finer oil filter and sometimes an oil cooler, before going through the engine’s oil passages and being dispersed to lubricate pistons, rings, springs, valve stems, and more.

Finally, the oil flows back to the oil sump and the cycle starts again. Continuously.

This video by Castrol illustrates the flow in vivid animation.

What is the Oil Filter for?

The oil filter is a component through which the oil flows and gets purified from the dirt particles it gathered cleaning the engine.

As the oil circulates, it cleans the engine, carrying away dust, dirt, and combustion residues to the oil filter to be captured.

If the filter weren’t there, the oil wouldn’t be cleaned, and it would actually spread the particles around all of the engine. The particles would void that cushioning oil film between parts, and get rubbed together with the various moving components, ruining them. The engine would soon break down.

Why does Motor Oil Need to be Regularly Changed?

In a way, motor oil protects the engine components by absorbing the damage.

Over time, however, the continuous friction, shearing forces, and heating break down the oil, wearing it out. When this happens, the oil starts behaving differently than expected, and it becomes less effective at lubricating the engine and absorbing heat.

In other words, over time the oil wears out and becomes less and less capable of protecting the engine.

That’s why, as the vehicle owner, you should change the oil in your engine regularly before it breaks down and causes major engine troubles.

How Often to Change Motor Oil

As with most things, your vehicle owner’s manual will be your guide.

Most manuals for modern vehicles recommend an oil change every 5000 to 7500 miles approximately. Sometimes they also specify a certain amount of time as an alternative, e.g., every 3 to 6 months, though that’s generally a less reliable metric (if you didn’t use your vehicle for the past 3 months it wouldn’t make much sense to change the oil).

However, you should absolutely take your driving style and habits into account.

Generally speaking,

  • if your vehicle is new, or
  • if your driving mainly consists of sessions of 20 minutes or more at fairly steady speeds (as opposed to stop-and-go traffic)

then you can probably increase a bit the mileage between oil changes without consequences.


  • if your vehicle has more than a couple of years, or
  • if your driving mostly consists of shorter than 20 minutes sessions or of stop-and-go traffic, or
  • if you frequently drive off-road or in particularly dusty/sandy environments

then you should definitely consider decreasing the time between oil changes, and perform more frequent maintenance (e.g., every 3000 or even 1000 miles).

That’s because if you aren’t making long trips at high, steady speeds (like you would on a highway) then your engine isn’t getting hot enough to boil off the condensation that accumulates in the system. And that can cause the oil to break down faster.

Additionally, most of the wear and tear on your engine occurs when you’re starting and stopping your car. And if you’re frequently driving in stop-and-go or heavy traffic conditions, then most of your driving is of the type that is very hard on your engine. More frequent oil changes will help minimize the damage.

In a nutshell, take your vehicle owner’s manual recommended schedule as the standard, and then apply the additional consideration above depending on your circumstances and habits.

Finally, always remember to change the oil filter too with any oil change.

This greatly simplifies things and reduces the amount of stuff you have to remember for your vehicle maintenance. In fact, that’s probably what the owner’s manual will recommend as well.

If you forget it, and the filter gets clogged up you’ll soon be in big trouble. Unfiltered oil will run through the system, and dirt particles will be spread around all of the engine wreaking havoc.

Plus, running new oil through a dirty old filter is debatable at best, don’t you think?!

How to Choose the Best Motor Oil for Your Vehicle

Again, as always… You guessed it, right? Always start with your vehicle’s owner manual!

However, there are many aspects to consider when choosing the best motor oil for your vehicle. Let’s go through them together.

Oil Viscosity Rating

The first and most important thing to know is your vehicle’s engine recommended oil viscosity rating. The best motor oil for your vehicle will be one with the recommended rating.

The viscosity rating can be found, like most things, in the vehicle owner’s manual. Alternatively, it is sometimes indicated right on the vehicle’s oil cap.

The rating is usually composed of two numbers separated by a W, e.g., 5W-30. Those numbers represent the oil viscosity, that is its thickness, its resistance to flow, when cold and when hot respectively.

Oil is generally thicker at lower temperatures and thinner at higher temperatures.

A visual explanation of motor oil viscosity rating

The numbers then represent how the oil behaves at cold and at hot temperatures.

The first number represents the behavior when cold (the W stands for winter by the way).

The second represents the behavior when hot (i.e., at operating condition).

The desired oil behavior depends on the vehicle engine, hence the recommended viscosity rating.

Generally speaking, thicker oil

  • provides greater protection for your engine parts
  • but it will be harder to get flowing, especially at the start when the engine is still cold, and could thus end up causing engine wear (because the engine will operate without oil temporarily)
  • because of the higher viscosity, it can act as a dampener for the engine motion, and can thus decrease fuel efficiency.

Thinner oil, on the other hand,

  • provides a thinner film between the moving motor parts, and thus less protection
  • but it will be easier to get flowing, especially at the start of the engine
  • because of the lower viscosity, it dampens less the engine motion and thus usually yields a higher fuel economy.

In synthesis, at lower temperatures, there’s a risk that the oil gets too thick and doesn’t flow as it should, whereas, at higher temperatures, there’s a risk that it gets too thin and doesn’t protect the engine as it should.

As far as the viscosity rating

  • the lower the first number, the better the oil will flow at lower temperatures (e.g., 5W oil will flow better than 10W oil at lower temperatures)
  • the higher the second number, the less thin the oil will be at higher temperatures and thus the better it will protect the engine (5W-40 oil will be less thin than 5W-30 oil at higher temperatures).

However, what we’ve just said is just for your personal education and curiosity. It is absolutely crucial to follow the oil viscosity recommended by the vehicle manufacturer.

Do not buy oil with a different rating out of your own volition. Maybe you’re in a colder climate and thus you may think you need a lower W rating. Or maybe you’re in a hotter climate and you’re considering a higher hot rating. Unless your vehicle owner’s manual has explicit guidelines on the matter, don’t. Just don’t. Simply use the recommended viscosity!

We’ll repeat that again: the desired oil behavior depends on the vehicle engine. Not all engines are created equal. Each engine expects a particular behavior from its oil. Your vehicle’s manufacturer obviously knows best. Hence, follow the recommended viscosity rating.

Types of Motor Oil

Not all motor oils are created in the same way. As we said, most motor oils are derived from crude oil. Then, depending on the refinement process and the additives in the mix, we have four general motor oil types:

Conventional Motor Oil

Conventional motor oil, aka regular motor oil or mineral motor oil, is the most commonly used type of oil. It is the standard new-car oil, ideal for light-duty, late-model cars with low to average mileage and a standard engine design.

Regular oil refinement process is generally shorter and simpler than other types. Thus, though other types may have better properties, conventional oil wins in the cost department.

The carmakers usually specify a 5W-20 or 5W-30 oil, particularly for lower temperatures, with a 10W-30 oil as optional, more indicated for higher ambient temperatures. These three ratings cover just about every light-duty vehicle on the road.

Full Synthetic Motor Oil

Synthetic motor oil is a lubricant consisting of chemical compounds that are artificially made, hence the name synthetic oil. The base material is still generally crude oil that is distilled and then heavily modified physically and chemically.

Synthetic oil refinement process is longer and more complex. And that’s why synthetic motor oil is more expensive but guarantees properties and benefits that are outstandingly superior.

Synthetic oils pass stringent special tests (indicated by their labeling) indicating they have superior, longer-lasting performance in all the critical areas, from viscosity index to protection against deposits.

They are ideal for vehicles that demand peak level performance and high levels of lubrication. They provide higher viscosity levels, resistance to oxidation and thermal breakdown, and help fight against oil sludge. Plus, they help improve fuel efficiency and can even increase a vehicle’s horsepower by reducing engine drag.

If you live in a climate with super cold winters or very hot summers, or if you use your vehicle for towing or hauling, or if you do a lot of short, stop-and-go traffic, synthetic oil may be the best type of oil for your vehicle.

Older vehicles could also benefit from synthetic oil, as it can help prevent or even dissolve harmful sludge build-up that some older engines seem to be prone to.

Synthetic Blend Motor Oil

Synthetic blend motor oil, as the name suggests, is a mixture of synthetic and conventional base oils, plus some additives, for extra resistance to oxidation and excellent low-temperature properties.

It has many of the characteristics of full synthetic oil, but at a generally lower price. Synthetic blends make it easy for drivers to make the switch from conventional to synthetic oil, which is why this type of oil is becoming increasingly popular among drivers.

It’s also a good middle-ground for drivers who want the added protection and performance of a synthetic oil, but might not be ready to foot the bill for a total switch to full synthetic oil.

High Mileage Motor Oil

High mileage motor oil is specially formulated for engines that have been around quite a bit. Generally speaking, for the average driver, a vehicle is classified as high mileage once it has run more than 75000 miles since it was built.

All engines, as they operate over time, get worn down. High mileage motor oil contains a variety of additional additives (compared to normal oil) that are particularly beneficial to older engines. In particular,

  • seal conditioners and additives that cause o-rings, gaskets, and seals to swell, thus resolving or preventing leaks and compression loss
  • detergents and dispersants designed to remove sludge from engines and prevent it in the first place.

This type of oil can help reduce oil consumption, minimize leaks and oil seepage, and can also help reduce smoke and emissions in older engines.

Combined with the usage of a good fuel system cleaner, a high mileage oil can restore an aging vehicle performance and keep it running smoothly for many more miles to come.

Do I Need Different Oil for Gasoline and Diesel Engines?

This one’s pretty simple. You want an oil that is appropriate for your type of engine.

If you have a gasoline engine, buy a gasoline motor oil. If you have a diesel engine, buy a diesel motor oil.

There are definitely differences between a motor oil designed for gasoline engines and one designed for diesel engines. There’s actually a link between the type of engine and the recommended viscosity. Since diesel engines’ operating temperatures are usually higher than gasoline engines’, diesel motor oil will usually have a higher recommended “hot viscosity rating”.

Similarly, the type and quantity of additives present may be different in a diesel engine oil compared to a gasoline oil.

Putting the wrong type of oil into your vehicle engine won’t generally have severe consequences. Do try to avoid it if you can, however, especially in the long run. Simply buy an oil with the recommended viscosity and compatible with your engine type.

Luckily, some newer oils are actually compatible both for gasoline and diesel engines. But you should always check if that’s the case.

It will be usually indicated explicitly on the oil jug. In the case that it isn’t, you can check the API oil certification labels on the back of the container.

API Rating and Certification Labels

The API donut and starburst labels

In the US, every container of reputable motor oil will have the API (American Petroleum Institute) donut and starburst labels indicating the service rating of the oil.

The API donut tells you if the oil meets the current S (for gasoline) or C (for diesel) service rating.

By the way, S stands for Spark ignition (i.e., gasoline) engine, while C stands for Compression ignition (i.e., diesel) engines. That should make it easier to remember!

The letter following the S or C indicates the “version” of the rating. Generally, the higher it is, the better the oil will be.

Additionally, newer versions are always “backward compatible” with older requirements. Thus, if, for example, your vehicle’s manual asks for an SJ rated oil, you can use an SJ, or SL, or SM, or SN oil and you will be fine.

As a recommendation, it’s better not to go lower than SJ, as these oils are obsolete and could cause damage to your engine.

The API donut also provides the SAE (Society of Automotive Engineers) viscosity rating of the oil and tells you if the oil has passed the Energy Conserving test.

Finally, the starburst symbol indicates that the oil has passed the tests listed for the S or C reported service level.

How to Check Your Vehicle’s Oil Level

It’s a great habit to check your vehicle’s oil once a month to make sure that there’s enough oil and that it isn’t badly contaminated for any reason.

The process is very simple and quick. Though most newer vehicles have electronic oil monitors, in the vast majority a proper oil dipstick is still present in the engine block for manual inspection.

Make sure the car is parked on level ground and, with most cars, that the engine is cold, so you don’t burn yourself on a hot engine part. With some cars, however, the automaker may recommend that the oil be checked after the engine has been warmed up for a while. Check the owner’s manual for precise instructions on the matter.

Check the video below for a quick tutorial.

When you check your oil, if it is dirty or smells of gasoline, it’s time for your oil to be changed.

Similarly, if the oil is clean but it only reaches the “Low” part of the dipstick, you’ll need to add more.

What does It Mean When the Oil Light is On on the Dashboard?

Well, simply put, it means you’d better hurry to a mechanic. Quickly!

The engine oil warning light

The oil light signals that there’s been a drop in oil pressure. And insufficient oil pressure means that oil isn’t flowing as it should inside the engine. The engine isn’t getting the lubrication it needs and this soon leads to self-destruction. And believe me, been there done that.

Get to the nearest mechanic or service station as soon as possible.

The most common reason for a drop in oil pressure is an insufficient amount of oil in the reservoir. You can easily check the oil level as we saw above to verify this.

If you’re running low on oil, you may easily fix the issue by topping it up if you have some with you. It’s actually a very good habit to always carry a small amount of spare oil in the trunk of your car.

However, even if you are able to fix the issue by replenishing the oil, a visit to the mechanic is still in order. A sudden oil shortage is always a very suspicious event and may indicate a more complex underlying issue. So have a professional take a look at your vehicle.

How to Change Motor Oil

Alright, now you know pretty much all you would ever need to know about motor oil from a functional point of view.

This final piece of the puzzle will complete your education and is actually deeply practical.

Of course we’re talking about changing your vehicle motor oil!

Most mechanics won’t charge you too much for an oil and filter change. However, they may be using cheap oil and very cheap oil filters.

That’s why it pays to bring your own oil and filter: you’ll be sure to be using the best motor oil and the best oil filter, and the service charges will be definitely lower.

Changing it completely by yourself is not complicated, but you better be ready to get your hands dirty.

If you do decide to go the DIY route and change it yourself, you’ll need:

Check out this video to learn how it’s done.

We absolutely recommend doing it yourself at least once if you can, to get a feeling for how it’s done and gain the experience.

If you do decide to stick to the DIY route then… Well, first of all, kudos to you! Secondly, we absolutely recommend investing in a good oil extractor to make the draining process a whole lot easier.


There you have it. Your ultimate guide to motor oil.

Motor oil is a fundamental component of practically all vehicles. And regular oil change is hands-down the most important maintenance activity you can perform on your vehicle.

You now have an exhaustive functional knowledge of motor oil. That gets you ahead of more than 99% of drivers out there and gives you the confidence to tackle this crucial maintenance task with ease, keeping your engine in top shape for years to come.

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