10W30 vs 5W30: Decoding the Numbers and Letters of Oil Types

10W30 vs 5W30

You’re about half a quart low, so you look in your owner’s manual to pick the right grade of oil from your auto parts store.

You read that the manufacturer recommends 10W30 motor oil. But why? What’s the difference between, say, 10W30 vs 5W30? What do those letters and numbers mean?

This article will attempt to demystify these cryptic ratings so you will always feel comfortable topping up the oil in your car.

What Is Motor Oil?

When comparing 10W30 vs 5W30 motor oil it is beneficial to start with the most basic question of all: What is motor oil?

Oil Revealed

Oil is a substance whose origins began millions of years ago. Oil is a fossil fuel that originally was a lot of tiny plants and animals, like algae and zooplankton, respectively.

As these little critters died, they settled to the bottom of what was then an ocean and were covered over many years with mud and silt.

As subsequent layers of fossils and mud layered on top of those that came before, temperature and pressure increased.

The hotter the temperature, the lighter the oil, and if the temperature went even higher, then natural gas was formed. So oil and natural gas are cousins.

Oil Is Not Just Oil

When you grab a plastic bottle of motor oil off the shelf, you’re getting a lot more than just oil.

Motor oil contains many different substances, including anti-wear additives, dispersants, detergents, corrosion inhibitors, anti-foam substances and, for multi-grade oils, viscosity index improvers.

There are more types of additives, but these are the main ones.

How Much of Motor Oil Is Additives?

How Much of Motor Oil Is Additives

Engine oil additives make up between 10% and 30% of your oil by volume. This may seem high, but additives do break down over time. We’ll discuss that in the next section.

Where Do the Additives Go?

Motor oil additives get used up over time. If they didn’t, you wouldn’t need to change your motor oil as often as you do.

So stick to your manufacturer’s recommended oil change instructions to be on the safe side. There are three ways that oil additives get used up: decomposition, separation and adsorption.

Decomposition means that with heat, pressure and time, some oil additives break their chemical bonds and become simpler substances, removing their effectiveness.

Separation means that certain additives attach to other substances and settle out of the oil, or are filtered out.

Adsorption is a chemical term meaning some additives chemically attach themselves to the metal parts of the engine.

Current Motor Oils/Filters

using high-quality motor oils and filters

Having said all that, additives are getting better and are becoming longer-lasting. Also, today’s premium oil filters can last for 10,000 miles.

So if you miss your oil change by a few hundred miles, don’t panic – as long as you’re using high-quality motor oils and filters.

Anti-Wear Additives

The basic function of motor oil is to reduce friction between moving parts in your car. Anti-wear additives enhance oil’s ability to reduce that friction even further.

The greater the reduction in friction, the longer your car’s moving parts will last.

The anti-wear additives used in motor oil are designated ‘AW.’ Heavier-duty anti-wear additives are designated ‘EP’ (Extreme Pressure) and are meant for heavier-duty uses such as use in gear boxes. Your transmission fluid contains an EP additive.

The most common AW additive for motor oil is zincdialkyl dithio phosphate (ZDDP), in case you’re into chemistry.


When your automobile engine burns gasoline, one substance formed is solid carbon substance known as soot or sludge.

The job of dispersants is to grab that sludge and hold it in suspension until it can be removed from the oil by an oil filter.


clean your engine by detergents

Detergents have two functions within your car’s engine. Their first function is what their name implies: to keep your engine clean.

Another byproduct of combustion are acids. Acids can damage your engine over time by wearing away the metal parts, particularly ones that are in constant motion, such as bearings and piston rings.

Detergents’ second function is to neutralize these acids so they can’t harm your car’s engine. They are usually chemical compounds consisting of magnesium and calcium.

Corrosion Inhibitors

As their name implies, corrosion inhibitors are there to protect your internal parts from oxidation (rust).

High temperatures and pressures make metals more prone to corrosion, so these substances are very necessary in today’s high-performance engines.

Anti-Foam Substances

Imagine your engine’s crankshaft spinning around just above your engine’s oil pan, splashing oil throughout the crankcase.

If that oil foams, then it can’t be picked up and distributed by the oil pump in the proper quantity needed to lubricate your engine.

Anti-foam additives help prevent your motor oil from foaming.

Viscosity Index Improvers

We’re going to discuss viscosity index improvers and reveal the secret of those numbers and letters in the next section.

But for now, think of viscosity as thickness. Honey has more viscosity than water, so honey flows more slowly and sticks to things more than water does.

Viscosity Index and Temperature 

Base Oil Viscosity Index

When oil comes into a refinery, it looks just one step above tar. In unrefined oil, called ‘crude,’ molecules of varying viscosity are all mixed together.

What a refinery does in very simplified terms is cook the oil, allowing the lighter components of the crude oil to evaporate out.

Those more volatile oil components are removed at various levels in the refinery tower. The resulting viscosity of the oil at each level is tested for its base viscosity index.

Why Change the Oil’s Viscosity?

In a word, temperature. The hotter an oil gets, the more its viscosity decreases, and vice versa.

If oil gets too hot, it becomes so thin that its lubrication capacity decreases, which can cause excessive wear and tear on your car’s engine.

If oil becomes too cold, then it can turn into a substance so thick that the oil pump can’t circulate it through the engine.

So in order to extend the base oil viscosity to account for changes in temperature, viscosity index improvers are necessary.

These viscosity improvers reduce the oil’s viscosity when it’s cold, and increase it when it’s hot.

The gauge of oil viscosity is found in the numbers and letters associated with the particular motor oil your manufacturer recommends for your car’s engine.

10W30 vs 5W30:

So let’s look at the numbers and letters in the comparison above. First, the letter ‘W.’ Many people assume that this stands for ‘Weight,’ but it does not.

The letter ‘W’ actually stands for ‘Winter.’ The reason the letter is there is to tell the consumer that the motor oil has been specifically blended to work in colder climates.

The numbers relate to the viscosity index of the additive-enhanced oil. With a 10W30 vs 5W30 oil, the 5W30has a lower viscosity when cold than a 10W30.

The number 30 on the end of the designation tells you that when the oil reaches full operating temperature, the viscosity is at 30.

Who Defined These Indices?

The body that defined 10W30 vs 5W30 motor oil viscosity and assigned the numbers and letters to them is the Society of Automotive Engineers (SAE).

It was founded in 1905 and is both a professional society and a standards-defining organization for the automotive industry.

When you see the numbers and letters that define the viscosity of your motor oil, somewhere on the label will be the letters ‘SAE.’

This shows the viscosity index ratings shown are in accordance with SAE standards.

But What Do the Numbers Mean?

We’re getting a little technical here, but since you asked: the number shown on the label are based on an SAE test. In that test, engineers place the oil in a tube of a specific size.

The number relates to how many seconds it takes for the oil of a particular viscosity to flow through the tube. So now you know.

Is There a Need to Choose Between 10W30 vs 5W30 Oil Types?

Choose Between 10W30 vs 5W30 Oil

Yes. Most auto manufacturers provide an oil viscosity chart that tells what oil type you should use based on how severe the winters are in your area.

The manufacturers chart temperature versus viscosity rating of oil. You would be wise to follow these instructions if you want to maximize the life of your engine.

As the Engine Ages

In an earlier automotive era, as an engine aged the oil channels increased in size. So as your engine aged the oil pressure went down over time because the oil pump had to move a greater volume of oil to keep your engine lubricated.

So it helped if you increased the oil viscosity in an older engine from, say, 5W30 to 10W40. Fortunately, with today’s engines, such a viscosity increase is no longer needed.


So now we have decoded the cryptic meaning behind 10W30 vs 5W30 motor oil viscosity ratings! We hope this will help put your mind at ease when you have to deal with future oil changes.    


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